Update GKE docs with visibility example

docs/tutorials/gke.md

@@ -123,6 +123,7 @@ spec:
         - --domain-filter=external-dns-test.gcp.zalan.do # will make ExternalDNS see only the hosted zones matching provided domain, omit to process all available hosted zones
         - --provider=google
 #        - --google-project=zalando-external-dns-test # Use this to specify a project different from the one external-dns is running inside
+        - --google-zone-visibility=private # Use this to filter to only zones with this visibility. Set to either 'public' or 'private'. Omitting will match public and private zones
         - --policy=upsert-only # would prevent ExternalDNS from deleting any records, omit to enable full synchronization
         - --registry=txt
         - --txt-owner-id=my-identifier