message: changed doctest to usage example
diff: @@ -1461,9 +1461,11 @@ class TensorBoard(Callback):
  [here](https://www.tensorflow.org/get_started/summaries_and_tensorboard).
  Example:
  - >>> tensorboard_callback = tf.keras.callbacks.TensorBoard(log_dir="./logs")
  - >>> model.fit(x_train, y_train, epochs=2, callbacks=[tensorboard_callback])
  - >>> #run the tensorboard command to view the visualizations
  + ```
  + tensorboard_callback = tf.keras.callbacks.TensorBoard(log_dir="./logs")
  + model.fit(x_train, y_train, epochs=2, callbacks=[tensorboard_callback])
  + #run the tensorboard command to view the visualizations
  + ```
  Arguments:
  log_dir: the path of the directory where to save the log files to be
  