Small update to instructions to make it work with the Package Manager

[bentwookie]

First, this plugin works wonderfully. Thank you for making it!

The only hiccup I ran into was Unity doesn't like the git path the instructions mention. The instructions say to enter git@github.com:donnerlab1/UnityGRPC.git into Package Manager, but I needed to enter https://github.com/donnerlab1/UnityGRPC.git.