doc: fix typo in volinfo man page

Reviewed-on: http://gerrit.openafs.org/10904
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: Ken Dreyer <ktdreyer@ktdreyer.com>
(cherry picked from commit 3a0c348d6ebc375f11d2bab70de9a00f5905fe94)

Change-Id: I8a39bdc1cb4bff509d54ef7c76d4b8735505c0e1
Reviewed-on: http://gerrit.openafs.org/10931
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>