message: Fix docstring typo: "GraphKey" -> "GraphKeys".
diff: @@ -381,7 +381,7 @@ class Optimizer(object):
  loss: A Tensor containing the value to minimize.
  var_list: Optional list or tuple of `tf.Variable` to update to minimize
  `loss`. Defaults to the list of variables collected in the graph
  - under the key `GraphKey.TRAINABLE_VARIABLES`.
  + under the key `GraphKeys.TRAINABLE_VARIABLES`.
  gate_gradients: How to gate the computation of gradients. Can be
  `GATE_NONE`, `GATE_OP`, or `GATE_GRAPH`.
  aggregation_method: Specifies the method used to combine gradient terms.
  