Fix usage example to specify a checkpoint to load.

Author: Alexander Gorban

research/attention_ocr/python/demo_inference.py

@@ -12,6 +12,7 @@
 
 Usage:
 python demo_inference.py --batch_size=32 \
+  --checkpoint=model.ckpt-399731\
   --image_path_pattern=./datasets/data/fsns/temp/fsns_train_%02d.png
 """
 import numpy as np