Set ClusterRole for roleRef in unauth webook example

Otherwise my webhooks don't work and I get failure replies to my
sent webhooks with messages like:

```
RBAC: role.rbac.authorization.k8s.io \"system:webhook\" not found.
```

Author: dustymabe

modules/unauthenticated-users-system-webhook.adoc

@@ -37,7 +37,7 @@ metadata:
   namespace: <namespace> <1>
 roleRef:
   apiGroup: rbac.authorization.k8s.io
-  kind: Role
+  kind: ClusterRole
   name: "system:webhook"
 subjects:
   - apiGroup: rbac.authorization.k8s.io