Bump assets/chezmoi.io/themes/book from 4540d66 to e939f40

Bumps [assets/chezmoi.io/themes/book](https://github.com/alex-shpak/hugo-book) from `4540d66` to `e939f40`.
- [Release notes](https://github.com/alex-shpak/hugo-book/releases)
- [Commits](alex-shpak/hugo-book@4540d66...e939f40)

---
updated-dependencies:
- dependency-name: assets/chezmoi.io/themes/book
  dependency-type: direct:production
...

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