message: Fix issue in the `Defun` docs
  This fix fixes a couple of typos in the `Defun` docs:
  `tf.Constant` -> `tf.constant`
diff: @@ -82,8 +82,8 @@ class Defun(object):
  return x + y, x - y
  # Building the graph.
  - a = tf.Constant([1.0])
  - b = tf.Constant([2.0])
  + a = tf.constant([1.0])
  + b = tf.constant([2.0])
  c, d = MyFunc(a, b, name='mycall')
  ```
  """
  