doc: pt_util manpage fixes

Add missing -help option.

Change-Id: I2edd7bcaf5b5799ba1216777200580df2cd9ca7f
Reviewed-on: http://gerrit.openafs.org/10409
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>