doc: remove undefined reference variable

Remove undefined reference variable 'common' from http2 API

PR-URL: #16106
Fixes: #16068
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http2.md

@@ -1902,7 +1902,9 @@ const req = client.request({
   ':authority': `localhost:${port}`
 });
 
-req.on('response', common.mustCall());
+req.on('response', (headers) => {
+  console.log(headers[http2.constants.HTTP2_HEADER_STATUS]);
+});
 let data = '';
 req.setEncoding('utf8');
 req.on('data', (chunk) => data += chunk);