fix: mathjax formula rendering

[Valastiri]

Summary

Fix rendered formula on takeoff page. Seems to be the way I had written it earlier was a little bit off. Renders fine locally but let's see if this new way works.

We should probably also figure out if there are any more "formulas" on site before merging so it's all in one go.

Location

• docs/pilots-corner/advanced-guides/flight-planning/takeoff-perf-calc.md

Discord username (if different from GitHub): valastiri