fix reference to example plugin service

.gitlab-ci/values.yaml

 replicaCount: 1
 containerPort: 8080
 manifests:
-  - https://core-plugin-example-${CI_COMMIT_REF_SLUG}.k3s.os.oxui.de/manifest.json
+  - https://frontend-plugin-example-${CI_COMMIT_REF_SLUG}.k3s.os.oxui.de/manifest.json
 ingress:
   enabled: true
   hosts: