doc: fix documentation of http2Stream.pushstream()

Improve documentation of callback signature of
http2Stream.pushStream() function to align with
the changes made in https://github.com/nodejs/node/pull/17406.

PR-URL: https://github.com/nodejs/node/pull/18258
Fixes: https://github.com/nodejs/node/issues/18198
Refs: https://github.com/nodejs/node/pull/17406


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>