correction of recursive call

Author: Maxence

Diff for: rest_framework_docs/api_endpoint.py

@@ -75,7 +75,7 @@ def __get_fields__(self, serializer):
                 if isinstance(sub_type(), serializers.Serializer):
                     item['fields'] = self.__get_fields__(sub_type)
             elif isinstance(field, serializers.Serializer):
-                item['fields'] = self.__get_fields__(field)
+                item['fields'] = self.__get_fields__(field.__class__)
             fields.append(item)
         return fields