Auto merge of #4921 - wking:doc/guide-rust, r=alexcrichton

doc/guide/dependencies: Add a 'rust' language marker

This had been added to the non-book docs in 2ad45a56 (#4455), despite the fact that the book didn't actually have that marker:

    $ git cat-file -p 2ad45a56:src/doc/book/src/guide/dependencies.md | grep -A2 'You can now use the'
    You can now use the `regex` library using `extern crate` in `main.rs`.

    ```

This is (along with the already-landed #4916), part of recovering from #4904.