fix docker-compose.yml for compose-with-grafana example

[benjamonnguyen]

What is the purpose of this change? What does it change?

Fixes docker-compose.yml in compose-with-grafana example.

Latest prometheus image needs to be provided a directory containing the prometheus.yml file.

Was the change discussed in an issue or in the forum before?

In a different repo: https://github.com/uber/cadence/issues/4406

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [ ] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [ ] I have run gofmt on the code in all commits
• [ ] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review