docs: fix typo in if.platform-system example

[henryiii]

Wrong regex symbol, seen in https://github.com/orgs/scikit-build/discussions/1115#discussioncomment-10479495

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.90%. Comparing base (203035d) to head (06c9ecf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #879   +/-   ##
=======================================
  Coverage   83.90%   83.90%           
=======================================
  Files          74       74           
  Lines        4387     4387           
=======================================
  Hits         3681     3681           
  Misses        706      706           

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