doc: fix color contrast issue in light mode

There is a color contrast issue with links in light mode on the API
highlight background. This only appears in modules.md. This change fixes
it without introducing other contrast issues in either light or dark
mode. Previously, the color contrast for some text on modules.html was
failing WCAG AA in light mode.

PR-URL: https://github.com/nodejs/node/pull/39660


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>