doc: use number which is bigger than 1024 as port in http2

With 80 port, user should run example with root permission.
To avoid such situation, use 8000 or 8443 as port number
in example of http2.

PR-URL: #46938
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>

Author: deokjinkim

doc/api/http2.md

@@ -354,7 +354,7 @@ server.on('stream', (stream, headers) => {
   stream.end('<h1>Hello World</h1>');
 });
 
-server.listen(80);
+server.listen(8000);
 ```
 
 Even though HTTP/2 streams and network sockets are not in a 1:1 correspondence,
@@ -2518,7 +2518,7 @@ server.on('stream', (stream, headers) => {
   stream.end('<h1>Hello World</h1>');
 });
 
-server.listen(80);
+server.listen(8000);
 ```
 
 ### `http2.createSecureServer(options[, onRequestHandler])`
@@ -2659,7 +2659,7 @@ server.on('stream', (stream, headers) => {
   stream.end('<h1>Hello World</h1>');
 });
 
-server.listen(80);
+server.listen(8443);
 ```
 
 ### `http2.connect(authority[, options][, listener])`