Update Learning-Environment-Executable.md (#2037)

* Update Learning-Environment-Executable.md

fixed issue when creating a build folder in the assets folder.  Refering to https://github.com/Unity-Technologies/ml-agents/issues/2033

* Update Learning-Environment-Executable.md

docs/Learning-Environment-Executable.md

    * In the File dialog, navigate to your ML-Agents directory.
    * Assign a file name and click **Save**.
    * (For Windows）With Unity 2018.1, it will ask you to select a folder instead
-     of a file name. Create a subfolder within the ML-Agents folder and select
+     of a file name. Create a subfolder within the root directory and select
-     subfolder's name as `env_name`.
+     subfolder's name as `env_name`. You cannot create builds in the Assets folder
 
 ![Build Window](images/mlagents-BuildWindow.png)