doc: fix http2stream.pushStream error doc

The old error code `ERR_HTTP2_STREAM_CLOSED` was removed in commit
0babd181 (pull request #17406), and the
testcase for http2stream.pushStream was changed accordingly, but the
documentation change was overlooked.

This commit fixes it and aligns the documentation with the testcase.

This is a part of the fixes hinted by #21470, which includes some tests
for error codes usage and documentation and enforces a stricter format.

Refs: https://github.com/nodejs/node/pull/21470
Refs: https://github.com/nodejs/node/pull/17406
Backport-PR-URL: https://github.com/nodejs/node/pull/22850
PR-URL: https://github.com/nodejs/node/pull/21487


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>