doc: use correct body font URLs
doc: use correct body font URLs

The CSS for the docs homepage (docs.rust-lang.org) was using the wrong
URL for the body font, resulting in the fallback serif font being used,
instead of the desired Source Serif Pro fonts.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust