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>