style.mdoc: Prefer Ql to Dq Li for literal display

The Li macros has been deprecated by mdoc some time ago. Recommend the
use of Ql instead.

Reviewed by: debdrup
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33232

(cherry picked from commit 5e077e08ef87126765b5a818def6cb99314ab192)