docs: clarify weekly schedule comment as 'every Sunday at 00:00 UTC' (#15)

The cron expression (0 0 * * 0) was already correct — GitHub Actions'
day-of-week field uses 0 for Sunday, confirmed against GitHub's own docs.
Only the comment's 'Saturday/Sunday midnight UTC' wording was ambiguous.

Author: daggerok

.github/workflows/update-data.yml

@@ -1,7 +1,7 @@
 name: Update WisdomTree ETF data
 
 on:
-  # Weekly refresh (Saturday/Sunday midnight UTC) plus manual runs.
+  # Weekly refresh (every Sunday at 00:00 UTC) plus manual runs.
   schedule:
     - cron: '0 0 * * 0'
   workflow_dispatch: