message: Tweak `tf.slice` documentation.
  Add the input argument (`foo`) to `tf.slice` example so that it actually works if it were run.
  Previously, the input argument was missing (perhaps implied), but the example is clearer with its inclusion.
diff: @@ -605,7 +605,7 @@ def slice(input_, begin, size, name=None):
  Note that @{tf.Tensor.__getitem__} is typically a more pythonic way to
  perform slices, as it allows you to write `foo[3:7, :-2]` instead of
  - `tf.slice([3, 0], [4, foo.get_shape()[1]-2])`.
  + `tf.slice(foo, [3, 0], [4, foo.get_shape()[1]-2])`.
  `begin` is zero-based; `size` is one-based. If `size[i]` is -1,
  all remaining elements in dimension i are included in the
  