Update basic guide instructions (#2741)

* Update basic guide instructions

The current guide is still about the old version of the 3DBall example

* Update Basic-Guide.md

docs/Basic-Guide.md

 to the training and which Brain is being trained. You can only perform training with
 a `Learning Brain`.
 
-1. Each platform agent needs an assigned `Learning Brain`.  In this example, each platform agent was created using a prefab.  To update all of the brains in each platform agent at once, you only need to update the platform agent prefab.  In the **Project** window, go to the `Assets/ML-Agents/Examples/3DBall/Prefabs` folder. Expand `Game` and click on the `Platform` prefab.  You should see the `Platform` prefab in the **Inspector** window.  In the **Project** window, drag the **3DBallLearning** Brain located in  `Assets/ML-Agents/Examples/3DBall/Brains` into the `Brain` property under `Ball 3D Agent (Script)` component in the **Inspector** window.  
+1. Each platform agent needs an assigned `Learning Brain`.  In this example, each platform agent was created using a prefab.  To update all of the brains in each platform agent at once, you only need to update the platform agent prefab.  In the **Project** window, go to the `Assets/ML-Agents/Examples/3DBall/Prefabs` folder. You should see the `3DBall` prefab in the **Inspector** window. Now expand the prefab and select `Agent` which should appear now on your **Inspector** window.  In the **Project** window, drag the **3DBallLearning** Brain located in  `Assets/ML-Agents/Examples/3DBall/Brains` into the `Brain` property under `Ball 3D Agent (Script)` component in the **Inspector** window.  
 
    **Note**: The Unity prefab system will modify all instances of the agent properties in your scene.  If the agent does not synchronize automatically with the prefab, you can hit the Revert button in the top of the **Inspector** window.