doc: update http2.md example code

`require('http');` should be `require('http2');`

PR-URL: https://github.com/nodejs/node/pull/14979


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>