Add .partial_update to ModelViewSet documentation

The ModelViewSet inherits from the UpdateModelMixin, which provides the `.partial_update` method. This should be reflected in the documentation.

docs/api-guide/viewsets.md

@@ -179,7 +179,7 @@ In order to use a `GenericViewSet` class you'll override the class and either mi
 
 The `ModelViewSet` class inherits from `GenericAPIView` and includes implementations for various actions, by mixing in the behavior of the various mixin classes.
 
-The actions provided by the `ModelViewSet` class are `.list()`, `.retrieve()`,  `.create()`, `.update()`, and `.destroy()`.
+The actions provided by the `ModelViewSet` class are `.list()`, `.retrieve()`,  `.create()`, `.update()`, `.partial_update()`, and `.destroy()`.
 
 #### Example