message: updated concatenate example + fix for zip example
diff: @@ -568,7 +568,7 @@ class DatasetV2(object):
  ```python
  a = Dataset.range(1, 4) # [ 1, 2, 3 ]
  b = Dataset.range(4, 7) # [ 4, 5, 6 ]
  - c = Dataset.from_tensor_slices(tf.reshape(tf.range(7,13), (3,2))) # [ [7, 8], [9, 10], [11, 12] ]
  + c = Dataset.from_tensor_slices([[7, 8], [9, 10], [11, 12]])
  d = Dataset.range(13, 15) # [ 13, 14 ]
  # The nested structure of the `datasets` argument determines the
  @@ -599,18 +599,16 @@ class DatasetV2(object):
  """Creates a `Dataset` by concatenating given dataset with this dataset.
  ```python
  - # NOTE: The following examples use `{ ... }` to represent the
  - # contents of a dataset.
  - a = { 1, 2, 3 }
  - b = { 4, 5, 6, 7 }
  + a = Dataset.range(1, 4) # [ 1, 2, 3 ]
  + b = Dataset.range(4, 8) # [ 4, 5, 6, 7 ]
  # Input dataset and dataset to be concatenated should have same
  # nested structures and output types.
  - # c = { (8, 9), (10, 11), (12, 13) }
  - # d = { 14.0, 15.0, 16.0 }
  + # c = Dataset.from_tensor_slices([[8, 9], [10, 11], [12, 13]])
  + # d = Dataset.from_tensor_slices([14.0, 15.0, 16.0])
  # a.concatenate(c) and a.concatenate(d) would result in error.
  - a.concatenate(b) == { 1, 2, 3, 4, 5, 6, 7 }
  + a.concatenate(b) # ==> [ 1, 2, 3, 4, 5, 6, 7 ]
  ```
  Args:
  