Fixed mistake in Model.from_db() example.

docs/ref/models/instances.txt

@@ -100,7 +100,7 @@ are loaded from the database::
         instance._state.adding = False
         instance._state.db = db
         # customization to store the original field values on the instance
-        instance._loaded_values = zip(field_names, values)
+        instance._loaded_values = dict(zip(field_names, values))
         return instance
 
     def save(self, *args, **kwargs):