doc: fix http2stream.pushStream error doc

The old error code `ERR_HTTP2_STREAM_CLOSED` was removed in commit
0babd18 (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: #21470
Refs: #17406
Backport-PR-URL: #22850
PR-URL: #21487
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: ChALkeR

doc/api/http2.md

@@ -3145,7 +3145,7 @@ added: v8.4.0
 Call [`http2stream.pushStream()`][] with the given headers, and wraps the
 given newly created [`Http2Stream`] on `Http2ServerResponse`.
 
-The callback will be called with an error with code `ERR_HTTP2_STREAM_CLOSED`
+The callback will be called with an error with code `ERR_HTTP2_INVALID_STREAM`
 if the stream is closed.
 
 ## Collecting HTTP/2 Performance Metrics