Update shop/templates/shop/cart.html

fix a `MultiValueDictKeyError at /cart/update/` when doing an `Update Shopping Cart` by providing a hidden id form field as mentioned in [Django docs](https://docs.djangoproject.com/en/1.4/topics/forms/modelforms/#using-the-formset-in-the-template). 

You can reproduce this bug by navigating to "Your cart" and trying to update some quantity on the formset.