Fix route manpage to show correct flush syntax

[rstone]

Summary

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.

[rgrimes]

I missed that, overall I think this man page could use a rework in general form, gather all the syntax up in the SYNOPSIS, but if we keep pecking away at it at least the errors can be removed.