valectl(8): explicitly mention network switches

Use the term "network switch" instead of the ambiguous term "switch".

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 a2f53837f0a8934cfcf7ba3c1fdfc93f217d3703)