elfctl: update man page example for 'no' prefix

Reported by: Mark Millard on freebsd-current@
Fixes: c763f99d11fd ("elfctl: prefix disable flags with "no"")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit dbc7364b1840ef3f36994952d085add5d161775d)