message: Minor change to default behaviour of `concatenate`.
diff: @@ -2497,7 +2497,7 @@ def concatenate(tensors, axis=-1):
  ```python
  >>> a = tf.constant([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
  >>> b = tf.constant([[10, 20, 30], [40, 50, 60], [70, 80, 90]])
  - >>> tf.keras.backend.concatenate((a, b), axis=1)
  + >>> tf.keras.backend.concatenate((a, b), axis=-1)
  <tf.Tensor: id=14, shape=(3, 6), dtype=int32, numpy=
  array([[ 1, 2, 3, 10, 20, 30],
  [ 4, 5, 6, 40, 50, 60],
  