Update example: Counter.subtract returns None, not self

Doc/library/collections.rst

@@ -120,6 +120,7 @@ For example::
             >>> c = Counter(a=4, b=2, c=0, d=-2)
             >>> d = Counter(a=1, b=2, c=3, d=4)
             >>> c.subtract(d)
+            >>> c
             Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6})
 
    The usual dictionary methods are available for :class:`Counter` objects