Fix typo

nmt/model.py

@@ -613,7 +613,7 @@ def _build_bidirectional_rnn(self, inputs, sequence_length,
                                num_bi_layers,
                                num_bi_residual_layers,
                                base_gpu=0):
-    """Create and call biddirectional RNN cells.
+    """Create and call bidirectional RNN cells.
 
     Args:
       num_residual_layers: Number of residual layers from top to bottom. For