Javadoc: fix examples using markdown instead of @code

[akarnokd]

In javadoc, backtick has no meaning and caused javadoc compilation errors:

https://travis-ci.org/akarnokd/RxJava2_9/jobs/469904725#L516

[codecov]

Codecov Report

Merging #6346 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6346      +/-   ##
============================================
- Coverage     98.27%   98.26%   -0.02%     
+ Complexity     6285     6283       -2     
============================================
  Files           672      672              
  Lines         44992    44992              
  Branches       6223     6223              
============================================
- Hits          44218    44212       -6     
- Misses          244      245       +1     
- Partials        530      535       +5

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	567 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	542 <0> (ø)	⬇️
...ternal/operators/observable/ObservablePublish.java	94.69% <0%> (-3.54%)	11% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	93.61% <0%> (-2.66%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ex/internal/operators/flowable/FlowableCreate.java	95.8% <0%> (-1.94%)	6% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-1.5%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	97.02% <0%> (-1.28%)	11% <0%> (+1%)
... and 22 more

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