Add missing tryfrom example

[GuillaumeGomez]

r? @rust-lang/docs

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:1d144326:start=1554404558230062618,finish=1554404646190781006,duration=87960718388
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
Setting environment variables from .travis.yml
---
travis_time:start:test_assembly
Check compiletest suite=assembly mode=assembly (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:13:52] 
[01:13:52] running 9 tests
[01:13:52] iiiiiiiii
[01:13:52] 
[01:13:52]  finished in 0.153
[01:13:52] travis_fold:end:test_assembly

---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:14:08] 
[01:14:08] running 121 tests
[01:14:33] .iiiii...i.....i..i...i..i.i.i..i.ii...i.....i..i....i..........iiii..........i...ii...i.......ii.i. 100/121
[01:14:37] i.i......iii.i.....ii
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[01:14:37]  finished in 29.710
[01:14:37] travis_fold:end:test_debuginfo

---
[01:25:51] 
[01:25:51]    Doc-tests core
[01:25:56] 
[01:25:56] running 2308 tests
[01:26:09] ....iiiii........................................................................................... 100/2308
[01:26:22] ......................................................................ii.....F...................... 200/2308
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---
[01:31:09] ---- convert.rs - convert::TryFrom (line 434) stdout ----
[01:31:09] error[E0405]: cannot find trait `TryFrom` in this scope
[01:31:09]  --> convert.rs:437:6
[01:31:09]   |
[01:31:09] 6 | impl TryFrom<i32> for SuperiorThanZero {
[01:31:09] help: possible candidate is found in another module, you can import it into scope
[01:31:09]   |
[01:31:09] 3 | use std::convert::TryFrom;
[01:31:09]   |
---
[01:31:09] 
[01:31:09] error: test failed, to rerun pass '--doc'
[01:31:09] 
[01:31:09] 
[01:31:09] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "core" "--" "--quiet"
[01:31:09] 
[01:31:09] 
[01:31:09] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:31:09] Build completed unsuccessfully in 0:29:11
[01:31:09] Build completed unsuccessfully in 0:29:11
[01:31:09] Makefile:48: recipe for target 'check' failed
[01:31:09] make: *** [check] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:252ab738
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Thu Apr  4 20:35:25 UTC 2019
---
travis_time:end:000cc741:start=1554410126743621859,finish=1554410126748968759,duration=5346900
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0e3186e8
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:015de621
travis_time:start:015de621
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:0afc8af0
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[GuillaumeGomez]

Updated!

[Centril]

r? @Centril r=me rollup with green travis

[GuillaumeGomez]

@bors: r=Centril rollup

[bors]

📌 Commit c386210 has been approved by Centril