sched: update the sched-domains debug documentation

SCHED_DOMAIN_DEBUG mentioned in the Documentation for sched-domains
for enabling sched-domains debugging doesn't exist anymore.

Update the documentation to reflect the correct way of enabling
sched-domain debugging.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>