'gcloud beta ml local train' args change

[amygdala]

AFAICT, gcloud beta ml local train args have had hyphens converted to underscores, and should now be:
train_data_paths, eval_data_paths, and output_path.

https://github.com/amygdala/tensorflow-workshop/blob/master/workshop_sections/mnist_series/cloudml/README.md#reading-tfrecords---exercise-3