Fix a small typo, filerset_class -> filterset_class (#762)

docs/filtering.rst

@@ -127,7 +127,7 @@ create your own ``FilterSet``. You can pass it directly as follows:
         all_animals = DjangoFilterConnectionField(AnimalNode,
                                                   filterset_class=AnimalFilter)
 
-You can also specify the ``FilterSet`` class using the ``filerset_class``
+You can also specify the ``FilterSet`` class using the ``filterset_class``
 parameter when defining your ``DjangoObjectType``, however, this can't be used
 in unison  with the ``filter_fields`` parameter:
 
@@ -218,4 +218,4 @@ with this set up, you can now order the users under group:
           xxx
         }
       }
-    }
+    }