Updating missed reference to python directory.

docs/Installation.md

     git clone https://github.com/Unity-Technologies/ml-agents.git
 
 The `MLAgentsSDK` directory in this repository contains the Unity Assets
-to add to your projects. The `python` directory contains the training code.
-Both directories are located at the root of the repository.
+to add to your projects. The `mlagents` directory contains the python package
+which provides trainers and a python API to interface with Unity.
 
 ## Install Python (with Dependencies)