Update the hugo book theme to the latest commit to fix build failure

"ERROR deprecated: .Sites.First was deprecated in Hugo v0.127.0 and will
be removed in Hugo 0.140.0. Use .Sites.Default instead." currently
breaks the build pipeline. It has been fixed in [1], but a new version
of the theme has not been released yet, so bump the submodule to the
latest commit, to get the fix.

[1] https://github.com/alex-shpak/hugo-book/pull/653

hugo-book @ 7c78a39c

-Subproject commit e104a11f42fbd069aa15606c5f01631b07d7528c
+Subproject commit 7c78a39c531aa2492ed7e92f2ce9dfb2c8c0d3fa