Improve readme to indicate official image locations

Correct image repos on Docker Hub:

```text
dockersamples/examplevotingapp_vote
dockersamples/examplevotingapp_worker
dockersamples/examplevotingapp_result
```

The alternate location is GitHub Container Registry:

```text
ghcr.io/dockersamples/example-voting-app-vote
ghcr.io/dockersamples/example-voting-app-worker
ghcr.io/dockersamples/example-voting-app-result
```