volinfo: fix bad format string

we need a format string; currently here we have none, which is no good

Reviewed-on: http://gerrit.openafs.org/5591
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 0676dc9e0c59e8aee3db5e0a7b4acf7229da7b88)

Change-Id: I665eabadee79c358562b831bf15d513e4f945f75
Reviewed-on: http://gerrit.openafs.org/11265
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>