message: Correct getting dataset.element_spec
  fixed a document error: dataset.element_spec
diff: @@ -410,7 +410,8 @@ class DatasetV2(tracking_base.Trackable, composite_tensor.CompositeTensor):
  def element_spec(self):
  """The type specification of an element of this dataset.
  - >>> dataset = tf.data.Dataset.from_tensor_slices([1, 2, 3]).element_spec
  + >>> dataset = tf.data.Dataset.from_tensor_slices([1, 2, 3])
  + >>> dataset.element_spec
  TensorSpec(shape=(), dtype=tf.int32, name=None)
  Returns:
  