route(8): Add an example how to print the routing tables

The manual page currently doesn't show an example how to print
the routing table, so add one and .Xr netstat while here.

PR: 231579
Reported by: Pekka Järvinen <pekka dot jarvinen at gmail dot com>
Reviewed by: debdrup
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D29702

(cherry picked from commit bc54f5f1cd5c01aa5f941234034839fb0b8f54ed)