Correct the update URL

[aanchal22]

Describe the problem
the URL of the update function is incorrect

To Reproduce
Go to the following APIs:
PUT /shopcarts/<int:customer_id>/<int:product_id>

Expected behavior
Needs to changed to:
PUT /shopcarts/<int:customer_id>/products/<int:product_id>

Additional context
We need to change the URL to be RESTful