Fix signal handler doc (#250)

If DEBUG is on and kwags is missing Django will raise the error: `ValueError: Signal receivers must accept keyword arguments (**kwargs).`

See https://github.com/django/django/blob/b1b26b37aff0c80d6abdf15c5ffdf0440a9a1d6a/django/dispatch/dispatcher.py#L87

Also parameters are called old_value and new_value.

Author: jwaschkau

docs/react_to_updates.rst

@@ -28,7 +28,7 @@ below.
 
     # yourapp/util.py
 
-    def notify_on_preference_update(sender, section, name, old, new):
+    def notify_on_preference_update(sender, section, name, old_value, new_value, **kwargs):
         print("Preference {} in section {} changed from {} to {}".format(
             name, section, old, new))