Bump themes/hugo-book from fb701f6 to 645c868 (#179)

Bumps [themes/hugo-book](https://github.com/alex-shpak/hugo-book) from `fb701f6` to `645c868`.
- [Release notes](https://github.com/alex-shpak/hugo-book/releases)
- [Commits](alex-shpak/hugo-book@fb701f6...645c868)

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

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