magento-test/pub/errors/default/503.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
?>

<h1>Service Temporarily Unavailable</h1>
<p>
    The server is temporarily unable to service your request due to maintenance downtime or capacity problems.
    Please try again later.
</p>