Sort link section alphabetically

This link section was already sorted but for a recent change (that
understandably probably preferred to keep the diff contained).  Let's
restore the sort order of the links here.

Author: traviscross

CONTRIBUTING.md

@@ -68,9 +68,9 @@ This should include links to any relevant information, such as the
 stabilization PR, the RFC, the tracking issue, and anything else that would be
 helpful for writing the documentation.
 
+[Authoring Guide]: docs/authoring.md
 [introduction]: src/introduction.md
 [issue tracker]: https://github.com/rust-lang/reference/issues
 [playpen]: https://play.rust-lang.org/
 [rust-lang/rust]: https://github.com/rust-lang/rust/
-[Authoring Guide]: docs/authoring.md
 [unstable]: https://doc.rust-lang.org/nightly/unstable-book/