message: concat to tf.concat for consistency
  Change concat to tf.concat for consistency in this doc and easier to copy and experiment with the code.
diff: @@ -1538,14 +1538,14 @@ def concat(values, axis, name="concat"):
  >>> t1 = [[1, 2, 3], [4, 5, 6]]
  >>> t2 = [[7, 8, 9], [10, 11, 12]]
  - >>> concat([t1, t2], 0)
  + >>> tf.concat([t1, t2], 0)
  <tf.Tensor: shape=(4, 3), dtype=int32, numpy=
  array([[ 1, 2, 3],
  [ 4, 5, 6],
  [ 7, 8, 9],
  [10, 11, 12]], dtype=int32)>
  - >>> concat([t1, t2], 1)
  + >>> tf.concat([t1, t2], 1)
  <tf.Tensor: shape=(2, 6), dtype=int32, numpy=
  array([[ 1, 2, 3, 7, 8, 9],
  [ 4, 5, 6, 10, 11, 12]], dtype=int32)>
  