public function admin_notice()
{
?>
<div class="notice notice-error">
<p><?php
printf(__('<strong>%1$s</strong>: To use this plugin, define the constant %2$s.', 'automattic-cron-control'), 'Cron Control', '<code>WP_CRON_CONTROL_SECRET</code>');
?>
</p>
</div>
<?php
}