Modify the notification message sent to admins when a new user requests approval to the site.

/**
* Modify the notification message
* $content the notification message
*/
function custom_notification_message() {
    $message = '<p>Hello</p>';
    $message .= '<p>' . __( '{username} ({user_email}) has requested a username at {sitename}', 'new-user-approve' ) . '</p>';
    $message .= '<p><a href="{site_url}">{sitename}</a></p>';
    $message .= '<p><a href="{admin_approve_url}">Approve or Deny User</a></p>';

    return $message;
}
add_filter('new_user_approve_request_approval_message_default', 'custom_notification_message');