Add error_reporting into config example file

Fixes the two errors found when going into administrator backend
( ! ) Notice: Undefined property: JConfig::$error_reporting in C:\wamp\www\jissues\administrator\includes\framework.php on line 49
( ! ) Notice: Undefined property: JConfig::$error_reporting in C:\wamp\www\jissues\administrator\includes\framework.php on line 76