doc: afsmonitor manpage fixes

Add missing option -debug.

Change-Id: I48b37cce536987f5cbf3905e1de70238e83664b5
Reviewed-on: http://gerrit.openafs.org/10404
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>