Fix installing.md for typo in GCE image names
@@ -12,8 +12,8 @@ latest version when possible):
* `ubuntu-2004-lts`, `ubuntu-1804-lts`, `ubuntu-1604-lts`, and `ubuntu-1404-lts`
* `debian-8`, `debian-7`
-* `centos-7`, `centos-6`
-* `rhel-7`, `rhel-6`
+* `centos-8`, `centos-7`
+* `rhel-7`
* `sles-12`
[Google Compute Engine]: https://cloud.google.com/compute/
