[CI] Update maintenance window comment

I updated the maintenance window so running terraform apply would actually work
given the restrictions put in place by GKE. I forgot to update the comment
though, so it ended up out of sync.

Author: boomanaiden154

premerge/gke_cluster/main.tf

@@ -20,7 +20,7 @@ resource "google_container_cluster" "llvm_premerge" {
     workload_pool = "llvm-premerge-checks.svc.id.goog"
   }
 
-  # We prefer that maintenance is done on weekends between 02:00 and 05:00
+  # We prefer that maintenance is done on weekends between 02:00 and 08:00
   # UTC when commit traffic is low to avoid interruptions.
   maintenance_policy {
     recurring_window {