docs: fix Record behavior in Awkward subclassing example

[Saransh-cpp]

Description

Coming from scikit-hep/coffea#1404

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Requests for the required change?
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.58%. Comparing base (a0cf1f7) to head (d9ca130).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #630   +/-   ##
=======================================
  Coverage   87.58%   87.58%           
=======================================
  Files          95       95           
  Lines       12058    12058           
=======================================
  Hits        10561    10561           
  Misses       1497     1497           

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