Typo fix in comment

Typo fix in comment

afsd -shutdown takes only a single dash, as with all cmd-style
options.

Improve the grammar a bit while we're here.

Reviewed-on: http://gerrit.openafs.org/12133
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 6db49a841784459cf583e914b3f2433ce1e70f4c)

Change-Id: Id3b12157c9d5ac3b9aeeba0c2af22fac33a2630f
Reviewed-on: https://gerrit.openafs.org/12197
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>