message: Change the link destination of cargo book contribution
  `src/doc` directory has `README.md` about Cargo documentation
  including description for contribution, while `src/doc/src` does not
  have such a document.
diff: @@ -27,4 +27,4 @@ The reference covers the details of various areas of Cargo.
  [rust]: https://www.rust-lang.org/
  [crates.io]: https://crates.io/
  -[GitHub]: https://github.com/rust-lang/cargo/tree/master/src/doc/src
  +[GitHub]: https://github.com/rust-lang/cargo/tree/master/src/doc
  