Disclaimer at top of reference has broken layout with latest mdbook

[frewsxcv]

https://doc.rust-lang.org/nightly/reference/introduction.html

[frewsxcv]

code:

• https://github.com/rust-lang-nursery/reference/blob/e6a5d5d10aa2fde0baed7b29bf672bd9f3af8962/src/theme/header.hbs#L1-L8
• https://github.com/rust-lang-nursery/reference/blob/aeeb04d5456ffa9394c04987483872ac9de6481a/src/theme/reference.css#L8-L46

[ehuss]

This is fixed in mdbook, but a new release has not yet been made. See #314.

[steveklabnik]

duplicate of #314