doc: label http.IncomingMessage as a Class

`Server`, `ServerResponse` etc. were marked as classes, this one class
was overlooked.

PR-URL: https://github.com/nodejs/node/pull/4589
Fixes: https://github.com/nodejs/node/issues/4576
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/http.markdown

@@ -788,7 +788,7 @@ should be used to determine the number of bytes in a given encoding.
 And Node.js does not check whether Content-Length and the length of the body
 which has been transmitted are equal or not.
 
-## http.IncomingMessage
+## Class: http.IncomingMessage
 
 An `IncomingMessage` object is created by [`http.Server`][] or
 [`http.ClientRequest`][] and passed as the first argument to the `'request'`