style.mdoc: fix list width alignment instructions

MFC after: 3 days
Reported by: imp
Reviewed by: mhorne, imp
Approved by: mhorne (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1530

(cherry picked from commit 4152ed2b999a0a8ccabfc085120e1c848d4f934a)