doc: fix typo in volinfo man page

Change-Id: I46e13bb879206fb561fe0f0d99a6ed412ab64629
Reviewed-on: http://gerrit.openafs.org/10904
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: Ken Dreyer <ktdreyer@ktdreyer.com>