message: fix variable reference in docs
diff: @@ -56,7 +56,7 @@ In Python, this scatter add operation would look like this:
  tensor = tf.ones([8], dtype=tf.int32)
  updated = tf.tensor_scatter_add(tensor, indices, updates)
  with tf.Session() as sess:
  - print(sess.run(scatter))
  + print(sess.run(updated))
  ```
  The resulting tensor would look like this:
  @@ -78,7 +78,7 @@ In Python, this scatter add operation would look like this:
  tensor = tf.ones([4, 4, 4])
  updated = tf.tensor_scatter_add(tensor, indices, updates)
  with tf.Session() as sess:
  - print(sess.run(scatter))
  + print(sess.run(updated))
  ```
  The resulting tensor would look like this:
  