message: Correct markdown for code segments in gradients_impl
diff: @@ -425,18 +425,22 @@ def gradients(ys,
  other things, this allows computation of partial derivatives as opposed to
  total derivatives. For example:
  + ```python
  a = tf.constant(0.)
  b = 2 * a
  g = tf.gradients(a + b, [a, b], stop_gradients=[a, b])
  + ```
  Here the partial derivatives `g` evaluate to `[1.0, 1.0]`, compared to the
  total derivatives `tf.gradients(a + b, [a, b])`, which take into account the
  influence of `a` on `b` and evaluate to `[3.0, 1.0]`. Note that the above is
  equivalent to:
  + ```python
  a = tf.stop_gradient(tf.constant(0.))
  b = tf.stop_gradient(2 * a)
  g = tf.gradients(a + b, [a, b])
  + ```
  `stop_gradients` provides a way of stopping gradient after the graph has
  already been constructed, as compared to `tf.stop_gradient` which is used
  