message: Fix doc formatting for tf.broadcast_to
diff: @@ -27,7 +27,8 @@ to broadcast a Tensor to a shape, it starts with the trailing dimensions,
  and works its way forward.
  For example,
  -```
  +
  +```python
  >>> x = tf.constant([1, 2, 3])
  >>> y = tf.broadcast_to(x, [3, 3])
  >>> sess.run(y)
  @@ -35,6 +36,7 @@ array([[1, 2, 3],
  [1, 2, 3],
  [1, 2, 3]], dtype=int32)
  ```
  +
  In the above example, the input Tensor with the shape of `[1, 3]`
  is broadcasted to output Tensor with shape of `[3, 3]`.
  END
  