Change `path` to `filename`

[mehuman]

As per https://grafana.com/docs/alloy/latest/reference/components/local.file/

PR Description

Updating the example to match the docs. Verified with a working example.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• [ ] CHANGELOG.md updated
• [X ] Documentation added
• [ ] Tests updated
• [ ] Config converters updated