Add missing Meta.fields in docs example. (#1155)

This example doesn't work as is, since a FilterSet with `Meta.model` requires `Meta.fields` (or `meta.exclude`).

docs/guide/tips.txt

@@ -207,6 +207,7 @@ behavior as an ``isnull`` filter.
 
         class Meta:
             model = MyModel
+            fields = []
 
 
 Filtering by relative times