fix a typo

[partev]

"added a a mean model" -> "added to a mean model"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.51%. Comparing base (8ff5a5e) to head (be7f0f0).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #763   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          74       74           
  Lines       15634    15634           
  Branches     1274     1274           
=======================================
  Hits        15558    15558           
  Misses         51       51           
  Partials       25       25           

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[bashtage]

Thanks.