Fix example about Default values (celery#4230)

Author: mozillazg

docs/internals/guide.rst

@@ -93,7 +93,7 @@ as this means that they can be set by either instantiation or inheritance.
         active = True
         serializer = 'json'
 
-        def __init__(self, serializer=None):
+        def __init__(self, serializer=None, active=None):
             self.serializer = serializer or self.serializer
 
             # must check for None when value can be false-y