doc: cpuset: Update the cpuset flag file

This patch is for modifying with correct cuset flag file.  We need to update
current manual for cpuset.  For example, before) cpus, cpu_exclusive, mems
after ) cpuset.cpus, cpuset.cpu_exclusive, cpuset.mems

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>