Incorrect property name makes example not work

Under heading `Overriding the initial queryset` the filter class is overridden which is documented as done by setting `filterset_class`. 
However under django_filters/rest_framework/backends.py in class `DjangoFilterBackend`, the method `get_filter_class` will use attribute `filter_class` instead. 

As a result, when trusting the documentation no filtering will take place at all.

docs/api-guide/filtering.md

@@ -127,7 +127,7 @@ Note that you can use both an overridden `.get_queryset()` and generic filtering
         """
         model = Product
         serializer_class = ProductSerializer
-        filterset_class = ProductFilter
+        filter_class = ProductFilter
 
         def get_queryset(self):
             user = self.request.user