args should agree with usage in function

docs/source/customization.rst

@@ -29,7 +29,7 @@ Example:
     class MySiteTree(SiteTree):
         """Custom tree handler to test deep customization abilities."""
 
-        def apply_hook(self, items, sender):
+        def apply_hook(self, tree_items, sender):
             # Suppose we want to process only menu child items.
             if tree_sender == 'menu.children':
                 # Lets add 'Hooked: ' to resolved titles of every item.