Translate Text using gettext filter/hook

/**
 * Translate Text using gettext filter/hook
 * As for where to paste this : http://wp-events-plugin.com/tutorials/how-to-safely-add-php-code-to-wordpress/
 */
function change_coupon_label( $translated_text, $text, $domain ) {
        switch ( $translated_text ) {
            case 'Coupon Code' :
                $translated_text = 'Promotion Code';
                break;
        }
    return $translated_text;
}
add_filter( 'gettext', 'change_coupon_label', 20, 3 );