Use "limit" instead of "paginate" for limit parameter.

[kayue]

Right now in API, if developer want to change the number of result per page, they have to use "paginate":

http://example.com/products?paginate=10

This PR will change it to default, which is "limit", a more common term for such parameter:

http://example.com/products?limit=10

Reference:

• http://docs.sylius.org/en/latest/api/products.html

[pjedrzejewski]

Thank you Ka Yue!