message: Fix set_difference doc
diff: @@ -247,7 +247,7 @@ def set_difference(a, b, aminusb=True, validate_indices=True):
  #
  # collections.OrderedDict([
  # ((0, 0, 0), 2),
  - # ((0, 0, 1), 3),
  + # ((0, 1, 0), 3),
  # ])
  ```
  