chore(issue-template): fix branch name

It was renamed to "main" from "master".

.github/ISSUE_TEMPLATE/tracking_issue.md

@@ -46,7 +46,7 @@ for larger features an implementation could be broken up into multiple PRs.
 
 [stabilization-guide]: https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr
 [doc-guide]: https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#documentation-prs
-[nightly-style-procedure]: https://github.com/rust-lang/style-team/blob/master/nightly-style-procedure.md 
+[nightly-style-procedure]: https://github.com/rust-lang/style-team/blob/main/nightly-style-procedure.md 
 [Style Guide]: https://github.com/rust-lang/rust/tree/master/src/doc/style-guide
 
 ### Unresolved Questions