vale(4): refer to the valectl(8) management tool

Signed-off-by: Tom Hukins <tom@FreeBSD.org>
Reviewed by: imp, mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1369

(cherry picked from commit 9a3fa77585e55e78004cdc9467b986968a77e8de)