doc: fix 'aborted' event documentation

The `'aborted'` event is only emitted on `http.IncomingMessage`
instances.

PR-URL: https://github.com/nodejs/node/pull/15471
Fixes: https://github.com/nodejs/node/issues/15259


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>