add_filter('wp_new_user_notification_email', 'codebykp_custom_user_notification_email', 10, 3);
function codebykp_custom_user_notification_email($wp_new_user_notification_email, $user, $blogname) {
    $wp_new_user_notification_email['subject'] = sprintf('Welcome to %s', $blogname);
    $wp_new_user_notification_email['message'] = 'Thank you for registering!';
    return $wp_new_user_notification_email;
}