doc: improve callback 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.
Fixes: https://github.com/nodejs/node/issues/18198 Refs: https://github.com/nodejs/node/pull/17406
Checklist
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documentation is changed or added -
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Affected core subsystem(s)
doc