Escape [sic] in docs (not an intra doc link) (hyperium#79)

Author: niklasf

src/common/referer.rs

@@ -5,7 +5,7 @@ use http::header::HeaderValue;
 /// `Referer` header, defined in
 /// [RFC7231](http://tools.ietf.org/html/rfc7231#section-5.5.2)
 ///
-/// The `Referer` [sic] header field allows the user agent to specify a
+/// The `Referer` \[sic\] header field allows the user agent to specify a
 /// URI reference for the resource from which the target URI was obtained
 /// (i.e., the "referrer", though the field name is misspelled). A user
 /// agent MUST NOT include the fragment and userinfo components of the