Fix math docs var domain

[brynpickering]

Fixes #652

@sstroemer linking you here (I've now also invited you as a member of the org - I'll start asking for your reviews once you've accepted that invite!)

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (1b7b04f) to head (3bcb979).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #674   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          26       26           
  Lines        3981     3981           
  Branches      837      768   -69     
=======================================
  Hits         3821     3821           
  Misses         70       70           
  Partials       90       90           

Files with missing lines	Coverage Δ
src/calliope/backend/latex_backend_model.py	96.85% <ø> (ø)