doc: fix comment about http2.createSecureServer

PR-URL: #15085
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http2.md

@@ -1549,7 +1549,7 @@ const options = {
   cert: fs.readFileSync('server-cert.pem')
 };
 
-// Create a plain-text HTTP/2 server
+// Create a secure HTTP/2 server
 const server = http2.createSecureServer(options);
 
 server.on('stream', (stream, headers) => {