message: update readme to remove `generation-num` arg to `train-dir`
diff: @@ -292,8 +292,7 @@ Run the training job:
  BOARD_SIZE=19 python3 main.py train-dir \
  estimator_working_dir
  gs://$BUCKET_NAME/data/training_chunks \
  - gs://$BUCKET_NAME/models/000001-somename \
  - --generation-num=1 \
  + gs://$BUCKET_NAME/models/000001-somename
  ```
  At the end of training, the latest checkpoint will be exported to the directory with the given name.
  