librustc: Fix poorly formatted doc in feature_gate

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