pkg/FreeBSD.conf: Fix comments to reflect -kmods

Now that this file lists two repositories, the text "to disable *this*
repository" no longer makes sense. Also, expand the example to list
how to disable both FreeBSD and FreeBSD-kmods repositories.

Reported by: Graham Perrin
MFC after: 3 days

(cherry picked from commit 955ab1a8f21ec1a662a87fc5e76e6d5690e09085)