sched: fix documentation reference for sched_min_granularity_ns

Impact: documentation fix

sched-design-CFS.txt wrongly references sched_granularity_ns sysctl,
as its name in fact is sched_min_granularity_ns.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>