generate_tfrecords command line format

[MAbdelatti]

Thank you very much for the efforts you've done in this example. I have a comment about the command line format you provided in the top of the generate_tfrecords.py file:

it should be: for training: python generate_tfrecord.py --csv_input=data/train_labels.csv --output_path=data/train.record --image_dir=images
and for testing: python generate_tfrecord.py --csv_input=data/test_labels.csv --output_path=data/test.record --image_dir=images

so that the image source directory is provided. Thanks!