highlight the correct words (#15659)

Author: weihanglo

src/doc/contrib/src/index.md

@@ -5,7 +5,7 @@ overview of how to contribute to Cargo, how to dive into the code, and how the
 testing infrastructure works.
 
 There are many ways to contribute, such as [helping other users], [filing
-issues], [improving the documentation], [triaging issues], [fixing bugs], and
+issues], improving [the documentation], [triaging issues], [fixing bugs], and
 working on [small] and [large features].
 
 If you have a general question about Cargo or its internals, feel free to ask
@@ -23,7 +23,7 @@ Please also read the [Rust Code of Conduct].
 [filing issues]: issues.md
 [rustup]: https://rust-lang.github.io/rustup/
 [git]: https://git-scm.com/
-[improving the documentation]: https://github.com/rust-lang/cargo/tree/master/src/doc
+[the documentation]: https://github.com/rust-lang/cargo/tree/master/src/doc
 [fixing bugs]: process/index.md#working-on-issues
 [small]: process/index.md#working-on-small-features
 [large features]: process/index.md#working-on-large-features