sysdecode_syscallnames.3: s/names/name/

The sysdecode_syscallname function was accidentally documented as being
plural. Move it to reflect it's actual name, and adjust all references.

PR: 278383
Reviewed by: ziaee
Reported by: mhorne (initial commit forgot to remove old)
Differential Revision: https://reviews.freebsd.org/D51002

(cherry picked from commit 739c4905dd5a35db8542b91cb46f04cc7b0484af)
(cherry picked from commit 4ba91e076ee84101112d8296785098ae31dac35e)