Change worker num to worker id

docs/Limitations-&-Common-Issues.md

 
 If you receive an exception `"Couldn't launch new environment because communication port {} is still in use. "`, you can change the worker number in the python script when calling 
 
-`UnityEnvironment(file_name=filename, worker_num=X)`
+`UnityEnvironment(file_name=filename, worker_id=X)`
 
 ### Mean reward : nan