Add missing import in /api-guide/viewsets/ example (encode#9235)

Author: mgaligniana

docs/api-guide/viewsets.md

@@ -311,7 +311,7 @@ You may need to provide custom `ViewSet` classes that do not have the full set o
 
 To create a base viewset class that provides `create`, `list` and `retrieve` operations, inherit from `GenericViewSet`, and mixin the required actions:
 
-    from rest_framework import mixins
+    from rest_framework import mixins, viewsets
 
     class CreateListRetrieveViewSet(mixins.CreateModelMixin,
                                     mixins.ListModelMixin,