Fix route manpage to show correct flush syntax

The current route(8) manpage shows that "flush" is an argument to
the optional -n flag, rather than a separate subcommand. Correct
this to properly show flush as a route subcommand.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Reviewed by: rgrimes
Differential Revision: https://reviews.freebsd.org/D14401