date.1: Lint with mandoc

Specifically, remove Tn macors, replace Li with Ql, and escape %N to
address date.1's rendering issues on man.freebsd.org.

PR: 290801
Reported by: jinwookjeongg@gmail.com
MFC after: 1 week
Sponsored by: Klara, Inc.

(cherry picked from commit e878ba8eea7206b3a435338c6eed0e4264e0ce14)