fix: wrong comment in postsolve basis

[Wronskode]

The comments in the case HighsBasisStatus::kUpper branch of
HighsPostsolveStack::SingletonRow::undo incorrectly referred to a
"tightened lower bound" and swapped the row bound origin, even though
this branch only handles the case where the column's upper bound was
tightened.

[jajhall]

OK

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.17%. Comparing base (ff64efa) to head (fad254e).
⚠️ Report is 18 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3119      +/-   ##
==========================================
+ Coverage   73.15%   73.17%   +0.01%     
==========================================
  Files         430      430              
  Lines      103937   103967      +30     
  Branches    16717    16718       +1     
==========================================
+ Hits        76039    76076      +37     
+ Misses      27622    27615       -7     
  Partials      276      276              

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