Bump chezmoi.io/themes/book from 179d444 to 7964687

Bumps [chezmoi.io/themes/book](https://github.com/alex-shpak/hugo-book) from `179d444` to `7964687`.
- [Release notes](https://github.com/alex-shpak/hugo-book/releases)
- [Commits](alex-shpak/hugo-book@179d444...7964687)

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