Updates websockets example.

doc/example-config-files/jitsi.example.com.example

 
     # xmpp websockets
     location /xmpp-websocket {
-        proxy_pass http://localhost:5280;
+        proxy_pass http://localhost:5280/xmpp-websocket;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection "upgrade";