add back the trailing comma

Author: jcrombez

form/create_custom_field_type.rst

@@ -35,7 +35,7 @@ for form fields, which is ``<BundleName>\Form\Type``. Make sure the field extend
                     'Standard Shipping' => 'standard',
                     'Expedited Shipping' => 'expedited',
                     'Priority Shipping' => 'priority',
-                )
+                ),
             ));
         }