tcpsso: add example to man page

Describe how to switch the TCP stack on all endpoints in the
LISTEN state.

Reviewed by: concussious.bugzilla_runbox.com
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46218

(cherry picked from commit 5c44e69d2f3d2404f39c552448173a9bd62bd651)