Fix add to cart bug in single-product.html

The append field is missing here, it cause a bug on the category view : you can't add twice the same product to the cart by clicking the "Add to cart" button. You can reproduce the bug on the Thelia demo here : http://demo.thelia.net/?view=category&locale=en_US&category_id=83, try to add twice the "Lexie" chair for instance.