doc: rename HTTP2 to HTTP/2

Previously, "HTTP/2" was strictly used to describe the protocol, and
HTTP2 the module. This distinction is deemed unnecessary, and
consistency between the two terms is enforced.

PR-URL: https://github.com/nodejs/node/pull/19603


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Chen Gang <gangc.cxy@foxmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>