sysrc.8: Rewrite examples for mdoc conformance

Tag SPDX while here. This is not a content change, so do not bump date.

Approved by: re (cperciva)
PR: 274164
MFC after: 3 seconds

(cherry picked from commit 3f0dd7724658126dcfde488a994cf7698353a82c)
(cherry picked from commit 9a6e6e9edc7eee4aeeb2f9f0e7be12eee656efaf)