use more simple language

Not obvious what "sort by" means

Author: tshepang

src/doc/rustc-dev-guide/src/contributing.md

@@ -346,7 +346,7 @@ function in the same way as other pull requests.
 [`src/doc`]: https://github.com/rust-lang/rust/tree/master/src/doc
 [std-root]: https://github.com/rust-lang/rust/blob/master/library/std/src/lib.rs#L1
 
-To find documentation-related issues, sort by the [A-docs label].
+To find documentation-related issues, use the [A-docs label].
 
 You can find documentation style guidelines in [RFC 1574].