chore: update cron to weekly and add frequency comment

Agent-Logs-Url: https://github.com/geocompx/geocompr/sessions/840d61e6-bed8-4133-ab8c-886a316fbbf0

Co-authored-by: Robinlovelace <1825120+Robinlovelace@users.noreply.github.com>

Author: Copilot

.github/workflows/scheduled.yaml

@@ -1,7 +1,7 @@
 name: Scheduled
 on:
   schedule:
-    - cron: '0 6 1 * *'
+    - cron: '0 6 * * 1'  # every Monday at 06:00 UTC
   workflow_dispatch:
 jobs:
   bookdown: