fix missing ',' after "mail_domain" parameter

config/config.sample.php

@@ -78,7 +78,7 @@ $CONFIG = array(
 "appstoreurl" => "http://api.apps.owncloud.com/v1",
 
 /* Domain name used by ownCloud for the sender mail address, e.g. no-reply@example.com */
-"mail_domain" => "example.com"
+"mail_domain" => "example.com",
 
 /* Enable SMTP class debugging */
 "mail_smtpdebug" => false,