Fix wrong book-count estimate in Matins responsory pool notes (66 -> 72)

"~66" was an eyeballed guess from a sort|uniq -c list, never actually
counted -- and coincidentally echoes the 66-book Protestant canon count,
a bad number to leave uncorrected in a Catholic office project's own
docs. The real count (grep -c distinct `book:` values across
data/hours/bible-plan/*.yml) is 72.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AGjUyhUZJaSjiniEmnLdak

src/data/hours/matins-responsories-by-book.yml

@@ -4,7 +4,7 @@
 # chapter/verse historically, so a book-level match is the right
 # granularity, not a gap to fill in later with exact citations.
 #
-# Seeded, not a full pool — 19 of the ~66 books actually used in the user's
+# Seeded, not a full pool — 19 of the 72 books actually used in the user's
 # own bible-plan (data/hours/bible-plan/*.yml), the rest deferred bulk
 # content (see TODO.md). Cycled in order
 # per-day when a Nocturn-1 reading's book has more than one candidate