fixed the aws doc (#1158)

* fixed the aws doc

* remove the cd instruction

docs/Training-on-Amazon-Web-Service.md

 
     ```sh
     git clone https://github.com/Unity-Technologies/ml-agents.git
-    cd ml-agents/python
+    cd ml-agents/ml-agents/
     pip3 install .
     ```
 
 8. Train the executable
 
     ```console
-    //cd into your ml-agents/python folder
-    python learn.py <your_env> --train
+    mlagents-learn <trainer-config-file> --env=<your_env> --train
     ```