message: Update document to use latest TF API - gfile has been moved to tf.io
diff: @@ -53,7 +53,7 @@ converter = tf.lite.TFLiteConverter.from_saved_model(export_dir)
  tflite_model = converter.convert()
  # Save the TF Lite model.
  -with tf.gfile.GFile('model.tflite', 'wb') as f:
  +with tf.io.gfile.GFile('model.tflite', 'wb') as f:
  f.write(tflite_model)
  ```
  @@ -125,7 +125,7 @@ converter = tf.lite.TFLiteConverter.from_concrete_functions([concrete_func])
  tflite_model = converter.convert()
  # Save the TF Lite model.
  -with tf.gfile.GFile('model.tflite', 'wb') as f:
  +with tf.io.gfile.GFile('model.tflite', 'wb') as f:
  f.write(tflite_model)
  ```
  