rename marginaleffects' s.value column name

[strengejacke]

Fixes #853

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.22%. Comparing base (7a44dbb) to head (0475c9c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #854      +/-   ##
==========================================
+ Coverage   55.09%   55.22%   +0.13%     
==========================================
  Files         125      125              
  Lines       15522    15523       +1     
==========================================
+ Hits         8552     8573      +21     
+ Misses       6970     6950      -20     

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