fix markup (rust-lang#1670)

Author: tshepang

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

@@ -280,7 +280,7 @@ from the source code itself (e.g. [`library/std/src/lib.rs`][std-root]). Documen
 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
+[std-root]: https://github.com/rust-lang/rust/blob/master/library/std/src/lib.rs#L1
 
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