Make comment in ptb_word_lm compatible with TF 1.0

tutorials/rnn/ptb/ptb_word_lm.py

@@ -147,7 +147,7 @@ class PTBModel(object):
     # The alternative version of the code below is:
     #
     # inputs = tf.unstack(inputs, num=num_steps, axis=1)
-    # outputs, state = tf.nn.rnn(cell, inputs,
+    # outputs, state = tf.contrib.rnn.static_rnn(cell, inputs,
     #                            initial_state=self._initial_state)
     outputs = []
     state = self._initial_state