style.mdoc: Li macro is undeprecated for some time

Previously, the mdoc linter warned against using Li as it was deprecated
upstream. However, upstream undeprecated it since last year. As usual
when it comes to style, the best thing to do is to follow the existing
style of the page. We have many manuals which prefer double quotes.

MFC after: 3 days
Citation: https://cvsweb.bsd.lv/mandoc/mdoc.7?rev=1.297
Differential Revision: https://reviews.freebsd.org/D55297

(cherry picked from commit fae551b76785d2357b09e2c022a92f410d871b9b)