message: Update datasets.md
  * Update datasets.md
  specifically I change Iterator into tf.data.Iterator
  * Update datasets.md
  I fixed the indentation according to your requesting.
diff: @@ -190,7 +190,7 @@ validation_dataset = tf.data.Dataset.range(50)
  # A reinitializable iterator is defined by its structure. We could use the
  # `output_types` and `output_shapes` properties of either `training_dataset`
  # or `validation_dataset` here, because they are compatible.
  -iterator = Iterator.from_structure(training_dataset.output_types,
  +iterator = tf.data.Iterator.from_structure(training_dataset.output_types,
  training_dataset.output_shapes)
  next_element = iterator.get_next()
  