Update Python-API.md

a tiny but import typo

docs/Python-API.md

 of your Unity environment is 3DBall.app, in python, run:
 
 ```python
-from mlagents.env import UnityEnvironment
+from mlagents.envs import UnityEnvironment
 env = UnityEnvironment(file_name="3DBall", worker_id=0, seed=1)
 ```