Fix typo in top-level README file

[bbenno]

The Stim project's URL contains a typo.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.16%. Comparing base (aa2b025) to head (ebacc23).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7125   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files        1093     1093           
  Lines       95432    95432           
=======================================
  Hits        93677    93677           
  Misses       1755     1755           

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