Fix highlight typo in source code documentation descriptions

[lemmingz]

Description

Fixes the hightlight typo in css descriptions.

[arcticicestudio]

Thanks for your contribution 🚀

I've checked the other sources and it would be nice if you could also fix the same typo in the LESS, SCSS and Stylus sources 👍

I think I'll need to add a writing linter to prevent lazy typos like this that get also copied over to other files 😆

[lemmingz]

✔️