Fix a3c wrong README.md (#7961)

Author: hougangliu

research/a3c_blogpost/README.md

@@ -3,4 +3,4 @@ In order to run this code, you will need the following prerequisites:
 
 * [OpenAI Gym](https://github.com/openai/gym) - `pip install gym`
 * [pyglet](https://bitbucket.org/pyglet/pyglet/wiki/Home) - `pip install pyglet` 
-* [TensorFlow](https://www.tensorflow.org/install/) - `pip install tensorflow`
+* [TensorFlow](https://www.tensorflow.org/install/) - `pip install tensorflow==v1.14.0`