Update books.py filename map for Solomon's Song

Update _BOOK_FILENAME_MAP to use "Solomon's Song" as the key and
point to the renamed file "solomons_song.json".

This fixes the book data loading issue where get_book_data() couldn't
find the book introduction, themes, and other metadata, causing the
page to fall back to generic auto-generated content.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

kjvstudy_org/books.py

@@ -35,7 +35,7 @@ _BOOK_FILENAME_MAP = {
     "Psalms": "psalms.json",
     "Proverbs": "proverbs.json",
     "Ecclesiastes": "ecclesiastes.json",
-    "Song of Solomon": "song_of_solomon.json",
+    "Solomon's Song": "solomons_song.json",
     "Isaiah": "isaiah.json",
     "Jeremiah": "jeremiah.json",
     "Lamentations": "lamentations.json",