frontend example reference to image is no longer valid

[gregorycox]

The link in here referencing v0.15.0-gke.8 is no longer accessible, v0.15.1-gke.0 is.

  containers:
  - name: frontend
    image: gke.gcr.io/prometheus-engine/frontend:v0.15.0-gke.8

[pintohutch]

@hsmatulisgoogle just assigned you to take a look

[pintohutch]

Also FYI @bernot-dev - do we have a presubmit that can verify this now?

[bernot-dev]

The presubmit checks the images in the actual manifests, but it does not currently cover example files. Perhaps we can add that.

[bernot-dev]

The correct tag in this case is v0.15.0-gke.12, until we get that fixed.