librustc: Fix poorly formatted doc in feature_gate

The #![feature(...)] line had been rendering in the docs as a header
because of Markdown syntax.

Author: tomjakubowski

src/librustc/front/feature_gate.rs

@@ -16,7 +16,7 @@
 //! enabled.
 //!
 //! Features are enabled in programs via the crate-level attributes of
-//! #![feature(...)] with a comma-separated list of features.
+//! `#![feature(...)]` with a comma-separated list of features.
 
 use middle::lint;