message: Update `flat_map` documentation so that the example is runnable.
  Note: Specifically `Dataset` is not a symbol in this example.
diff: @@ -1874,7 +1874,8 @@ name=None))
  >>> dataset = tf.data.Dataset.from_tensor_slices(
  ... [[1, 2, 3], [4, 5, 6], [7, 8, 9]])
  - >>> dataset = dataset.flat_map(lambda x: Dataset.from_tensor_slices(x))
  + >>> dataset = dataset.flat_map(
  + ... lambda x: tf.data.Dataset.from_tensor_slices(x))
  >>> list(dataset.as_numpy_iterator())
  [1, 2, 3, 4, 5, 6, 7, 8, 9]
  