CI: Ignore book/404.html with link checker

Author: alexdewar

.github/workflows/check-docs.yml

@@ -19,4 +19,4 @@ jobs:
         id: lychee
         uses: lycheeverse/lychee-action@v2
         with:
-          args: "--verbose --no-progress --accept 200,203,503 --exclude-path ^target './**/*.md' './**/*.html'"
+          args: "--verbose --no-progress --accept 200,203,503 --exclude-path ^target --exclude-path ^book/404.html$ './**/*.md' './**/*.html'"