Update doc/installation/cloud/gke.md Changed the description of the ADMIN_USER...

Updated doc/installation/cloud/gke.md. Changed the description of the ADMIN_USER variable from "The user to assign cluster-admin access to during setup" to "Grant Tiller, the server side of Helm, the cluster-admin role in your cluster". Since "The user to assign cluster-admin access to during setup" is difficult to understand to most users, I updated it with an easier to understand text. Also, I am new to gitlab, so please feel free to reach out with guidance on how to improve my contributions. Looking forward to being part of the community. Thank you.