Fix the URL for the blog about the book.

Jekyll generates blog entries using their categories as names.  The
first category for this was 'book', but that's the directory used for
the book and so GitHub Pages silently clobbered that directory.

_posts/2023-11-06-cheriot-programmers-guide.markdown

@@ -2,7 +2,7 @@
 layout: post
 title:  "CHERIoT Programmers' Guide drafts online"
 date:   2023-11-06T15:49+00:00
-categories: book guide
+categories: guide documentation
 ---
 
 You might have noticed a link in the header to the [CHERIoT Programmers' Guide](https://cheriot.org/book).