modsign: Fix documentation on module signing enforcement parameter.

Modify the documentation to match the actual parameter as implemented in
kernel/module.c:273.

Signed-off-by: James Johnston <johnstonj.public@codenest.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>