Making Required Div Label Activation Setting Same as Default Value

The [default value of the required div label setting](https://github.com/wpninjas/ninja-forms/blob/929d4c0a955b48ef073b225595c9a29c896059ac/includes/display/form/required-label.php#L16) is different from the label created on plugin activation. I simply added a period to make them consistent. This will be nice for any users that have to translate Ninja Forms as it's one less string to translate.