rustbook: use a relative @import path to ensure it works offline.

The URL previously caused the book to require an internet connection to
get the full styling, this should mean no internet is required.

src/rustbook/css.rs

@@ -11,7 +11,7 @@
 // The rust-book CSS in string form.
 
 pub static STYLE: &'static str = r#"
-@import url("//static.rust-lang.org/doc/master/rust.css");
+@import url("../rust.css");
 
 body {
     max-width:none;