fix: remove stale "contains" docs on rfc-index sync

[jennifer-richards]

Due to a typo / variable naming issue, a document no longer contained by a subseries doc was removed from the wrong subseries_doc.

Fixes #7244

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.71%. Comparing base (18cea10) to head (ff3514e).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9049      +/-   ##
==========================================
- Coverage   88.71%   88.71%   -0.01%     
==========================================
  Files         320      321       +1     
  Lines       41793    41822      +29     
==========================================
+ Hits        37078    37102      +24     
- Misses       4715     4720       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.