Syntax highlighting not working anymore for BibTeX

[Antoine-Zimmermann]

A solution that works: replace lines 28-30 of book.php with:

    <script src="https://cdn.jsdelivr.net/npm/prismjs-bibtex@2.1.0/prism-bibtex.js"
	        integrity="sha256-A5GMUmGHpY8mVpfcaRLQFeHtmdjZLumKBOMpf81FXX0="
	        crossorigin="anonymous" referrerpolicy="no-referrer">

[Antoine-Zimmermann]

This is addressed by PR #73