ejabberd.yml.example: Use non-standard STUN port

STUN via UDP can easily be abused for reflection/amplification DDoS
attacks.  Suggest a non-standard port to make it harder for attackers to
discover the service.

Modern XMPP clients discover the port via XEP-0215, so there's no
advantage in sticking to the standard port.

ejabberd.yml.example

@@ -67,7 +67,7 @@ listen:
       /admin: ejabberd_web_admin
       /.well-known/acme-challenge: ejabberd_acme
   -
-    port: 3478
+    port: 5478
     ip: "::"
     transport: udp
     module: ejabberd_stun