doc: fix mistake in http2stream.respondWithFile.

http2stream.respondWithFile api has changed since 8.5.0 with the
addition of the onError option. In the first code example an
onError function is implemented but never used, fix this
mistake.

Add a description to have more informations when onError is triggered.

PR-URL: https://github.com/nodejs/node/pull/15501
Fixes: https://github.com/nodejs/node/issues/15390


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>