Fix docs warning about get_html_theme_path.

[bdice]

Description

This fixes a failure in docs builds that comes from a warning.

See https://github.com/rapidsai/rmm/issues/1705 and https://github.com/rapidsai/rmm/pull/1706 for more information.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The cmake-format.json is up to date with these changes.
• [ ] I have added new files under rapids-cmake/
  • [ ] I have added include guards (include_guard(GLOBAL))
  • [ ] I have added the associated docs/ rst file and update the api.rst

[bdice]

/merge