Bump themes/hugo-book from 115a941 to 9405c4c (#227)

Bumps [themes/hugo-book](https://github.com/alex-shpak/hugo-book) from `115a941` to `9405c4c`.
- [Release notes](https://github.com/alex-shpak/hugo-book/releases)
- [Commits](alex-shpak/hugo-book@115a941...9405c4c)

---
updated-dependencies:
- dependency-name: themes/hugo-book
  dependency-version: 9405c4c3d7c7d81ae5be06589982691efedac3ea
  dependency-type: direct:production
...

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Author: dependabot[bot]