Change QuickStart Installation Link to point at correct updated unity plugin and not the old deprecated unity plugin

[zhenwei2972]

Hi would like to request that the quick start guide be updates with this link
https://github.com/ValveSoftware/unity-xr-plugin/releases/tag/v1.1.4
instead of the one that the installation is currently pointing to, as that was old and has not been updated, causing some frustration for users trying to follow the quickstart guide.