rustdoc: Links are styled as if they are Rust items

See [here](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/#modules) for example:

![image](https://user-images.githubusercontent.com/37223377/95005414-0c5f8600-05ad-11eb-9a6c-f8d4e02ee21c.png)

The links in the right column should be serif, not sans-serif.