Corrected typo in FilterSet reference. (#1301)

docs/ref/filterset.txt

@@ -71,7 +71,7 @@ include both transforms and lookups, as detailed in the `lookup reference`__.
 
 __ https://docs.djangoproject.com/en/stable/ref/models/lookups/#module-django.db.models.lookups
 
-Note that it is **not** necessary to included declared filters in a ``fields``
+Note that it is **not** necessary to include declared filters in a ``fields``
 list - doing so will have no effect - and including declarative aliases in a
 ``fields`` dict will raise an error.