Make clear meaning of <env_name>

python/README.md

 
 ## Training PPO directly
 
-To train using PPO without the notebook, run:
-
-`python3 ppo.py <env_name> --train`
+To train using PPO without the notebook, run: `python3 ppo.py <env_name> --train`
-For a list of additional hyperparameters, run:
+Where `<env_name>` corresponds to the name of the built Unity environment.
-`python3 ppo.py --help`
+For a list of additional hyperparameters, run: `python3 ppo.py --help`
 
 ## Using Python API
 See this [documentation](../docs/Unity-Agents---Python-API.md) for a detailed description of the functions and uses of the Python API.