message: updated map doc example
diff: @@ -932,17 +932,17 @@ class DatasetV2(object):
  For example:
  ```python
  - # NOTE: The following examples use `{ ... }` to represent the
  - # contents of a dataset.
  - a = { 1, 2, 3, 4, 5 }
  + a = Dataset.range(1, 6) # [ 1, 2, 3, 4, 5 ]
  - a.map(lambda x: x + 1) = { 2, 3, 4, 5, 6 }
  + a.map(lambda x: x + 1) # ==> [ 2, 3, 4, 5, 6 ]
  ```
  The input signature of `map_func` is determined by the structure of each
  element in this dataset. For example:
  ```python
  + # NOTE: The following examples use `{ ... }` to represent the
  + # contents of a dataset.
  # Each element is a `tf.Tensor` object.
  a = { 1, 2, 3, 4, 5 }
  # `map_func` takes a single argument of type `tf.Tensor` with the same
  