message: Fix typo in wrapper.py
  Fix a typo, from "backard_layer" to "backward_layer"
  in an example of bidirectional custom backward layer.
diff: @@ -394,7 +394,7 @@ class Bidirectional(Wrapper):
  # With custom backward layer
  model = Sequential()
  forward_layer = LSTM(10, return_sequences=True)
  - backard_layer = LSTM(10, activation='relu', return_sequences=True,
  + backward_layer = LSTM(10, activation='relu', return_sequences=True,
  go_backwards=True)
  model.add(Bidirectional(forward_layer, backward_layer=backward_layer,
  input_shape=(5, 10)))
  