fix missing units in avg_disk_commit_time

Change-Id: Ieb79dd7c10d0633b9c5e513e2fab693b83b3b06b
Reviewed-on: http://review.couchbase.org/30799
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>