docs: typo fix in config link to official documentation

[dali-does]

Summary

This PR fixes a typo in the configuration docs linking to the date tree footnote in the official orgmode.org manual.

Related Issues

Not applicable.

Changes

• .hml -> .html in the link to https://orgmode.org/manual/Template-elements.html#FOOT84

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• [-] Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Thanks!