cpuset: Fix documentation punctuation

Fix cpusets.txt documentation punctuation.

Signed-off-by: Greg Thelen <gthelen@google.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>