Update usage.rst in document

[fanrenkong]

In the "Using Django Ratelimit" section of the original document, it is explained that the decorator can limit HTTP methods. ratelimt.All represents all HTTP methods, and ratelimit.UNSAFE represents ('POST', 'PUT', 'PATCH', 'DELETE'). However, when upgrading from version 3.x to 4.0, imports should be renamed from from ratelimit to from django_ratelimit, so the document needs to be updated.