Use implicit reference links for shorter lines

(As suggested by jyn514.)

Author: follower

src/contributing.md

@@ -373,12 +373,12 @@ the submodule to. Running `./x.py build` should work now.
 ## Writing Documentation
 
 Documentation improvements are very welcome. The source of `doc.rust-lang.org`
-is located in [`src/doc`][srcdoc] in the tree, and standard API documentation is generated
-from the source code itself (e.g. [`lib.rs`][librs]). Documentation pull requests function
+is located in [`src/doc`] in the tree, and standard API documentation is generated
+from the source code itself (e.g. [`lib.rs`]). Documentation pull requests function
 in the same way as other pull requests.
 
-[srcdoc]: https://github.com/rust-lang/rust/tree/master/src/doc
-[librs]: https://github.com/rust-lang/rust/blob/master/library/std/src/lib.rs#L1
+[`src/doc`]: https://github.com/rust-lang/rust/tree/master/src/doc
+[`lib.rs`]: https://github.com/rust-lang/rust/blob/master/library/std/src/lib.rs#L1
 
 To find documentation-related issues, sort by the [T-doc label][tdoc].