doc: update http2.md example code

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

PR-URL: nodejs/node#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>

doc/api/http2.md

@@ -893,7 +893,7 @@ invoked with two arguments: an Object containing the received
 For example:
 
 ```js
-const http2 = require('http');
+const http2 = require('http2');
 const client = http2.connect('https://localhost');
 const req = client.request({ ':path': '/' });
 req.on('response', (headers, flags) => {