doc: fix typo on ka-forwarder man page

Change-Id: I9cedc1ee2c3fac249ad5b2c66c5e0eef6d5d570c
Reviewed-on: http://gerrit.openafs.org/10873
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>