DataFrame: Fix NOTE section and error in an output cell (#3766)
DataFrame: Fix NOTE section and error in an output cell (#3766)

* Fix an error message seen in one of the output cells in the tutorial notebook
  at https://www.tensorflow.org/tensorboard/dataframe_api#saving_the_dataframe_as_csv
* This error was caused by out-of-order execution of the notebook
* See screenshot of the error: ![image](https://user-images.githubusercontent.com/16824702/85901311-8d013000-b7cf-11ea-8fa2-6793befa17b0.png)
* Also in this change: Fix the formatting of the "NOTE" section.

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard