message: Fix typo and format for tf.math.unsorted_segment_sum
diff: @@ -38,7 +38,7 @@ added to the sum of the segment.
  ``` python
  c = tf.constant([[1,2,3,4], [5,6,7,8], [4,3,2,1]])
  -tf.unsorted_segment_sum(c, tf.constant([0, 1, 0]), num_segments=2)
  +tf.math.unsorted_segment_sum(c, tf.constant([0, 1, 0]), num_segments=2)
  # ==> [[ 5, 5, 5, 5],
  # [5, 6, 7, 8]]
  ```
  