Update example to be compatible to 2.x

Author: yifeif

README.md

@@ -61,7 +61,7 @@ pip install artifacts/*.whl
 Then test out the pip package
 ```bash
 cd ..
-python -c "import tensorflow as tf;import tensorflow_custom_ops as custom_ops_module;print(custom_ops_module.zero_out([[1,2], [3,4]]).eval(session=tf.Session()))"
+python -c "import tensorflow as tf;import tensorflow_zero_out;print(tensorflow_zero_out.zero_out([[1,2], [3,4]]))"
 ```
 And you should see the op zeroed out all input elements except the first one:
 ```bash