auto merge of #15097 : tomjakubowski/rust/fix-feature-gate-docs, r=sfackler

See http://static.rust-lang.org/doc/master/rustc/front/feature_gate/index.html for the problem this fixes.

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;