Fix reference in _Vertices._Cpo.operator()

[akrzemi1]

The if constexpr branch for _Strat_ref == _St_ref::_Auto_eval does not compile, as in the following example:
https://godbolt.org/z/Gqxfxhc49

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8fa358b) 98.10% compared to head (c9c18ff) 98.10%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   98.10%   98.10%   -0.01%     
==========================================
  Files          37       37              
  Lines        3796     3795       -1     
==========================================
- Hits         3724     3723       -1     
  Misses         72       72              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pratzl]

Thanks!