update doc comment

docs/Installation.md

 project by:
 
 1. navigating to the menu `Window` -> `Package Manager`.
-1. In the package manager window click on the `+` button.
+1. In the package manager window click on the `+` button on the top left of the packages list).
-
-**NOTE:** In Unity 2018.4 the `+` button is on the bottom right of the packages
-list, and in Unity 2019.3 it's on the top left of the packages list.
 
 <p align="center">
   <img src="images/unity_package_manager_window.png"