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.
Reported by: Pekka Järvinen <pekka dot jarvinen at gmail dot com>
Reviewed by: debdrup
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D29702