doc: fix port of destination server is not defined in http2

`port` variable is used, but it's not defined in client code.
So need to specify destination server(not proxy)'s port(8000)
in client code.

Refs: https://github.com/nodejs/node/blob/main/doc/api/http2.md#supporting-the-connect-method
PR-URL: #46940
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

doc/api/http2.md

@@ -3126,7 +3126,7 @@ const client = http2.connect('http://localhost:8001');
 // for CONNECT requests or an error will be thrown.
 const req = client.request({
   ':method': 'CONNECT',
-  ':authority': `localhost:${port}`,
+  ':authority': 'localhost:8000',
 });
 
 req.on('response', (headers) => {