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

This had been added to the non-book docs in 2ad45a5 ([doc] Sync back
doc/book changes into old docs, 2017-08-31, #4455), despite the fact
that the book didn't actually have that marker:

  $ git cat-file -p 2ad45a5: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`.

  ```

src/doc/src/guide/dependencies.md

@@ -70,7 +70,7 @@ we choose to `cargo update`.
 
 You can now use the `regex` library using `extern crate` in `main.rs`.
 
-```
+```rust
 extern crate regex;
 
 use regex::Regex;