fix: restore LaTeX subscripts on Marton constant page

[Chessing234]

Summary

• Restore proper _ subscripts inside math mode on the 18a (Marton/PFR) constant page
• Reverts the regression from fix: escape inline LaTeX subscripts on Marton constant page #98 where \_ was used inside $...$, which renders literally instead of as subscripts

Test plan

• Build the Jekyll site and confirm $\mathbb F_2^n$ renders correctly in the description and references

Made with Cursor

[Chessing234]

Same issue as #116 on 33a — #98 escaped underscores inside math delimiters, so F₂ showed up with a backslash.