fix a header in changelog

[longility]

This is very simple and straightforward as far as I can tell so not sure if I need to do much here. Let me know.

Summary

Inconsistent header styling in changelog

Test plan

None

[codecov-io]

Codecov Report

Merging #5840 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5840   +/-   ##
=======================================
  Coverage   63.74%   63.74%           
=======================================
  Files         216      216           
  Lines        7941     7941           
  Branches        4        3    -1     
=======================================
  Hits         5062     5062           
  Misses       2878     2878           
  Partials        1        1

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[rickhanlonii]

Thanks!

[github-actions]

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