Fixing manpage for vos addsite -valid

Fixing manpage for vos addsite -valid

The -valid switch to vos addsite doesn't take an argument, and it is
optional. Correcting documentation to reflect that.

Reviewed-on: http://gerrit.openafs.org/11
Verified-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>
(cherry picked from commit 7488aeef5cfd58935fe4a6a07810c8d757dd98a3)
Reviewed-on: http://gerrit.openafs.org/271
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>