sched: refresh MAINTAINERS entry

Peter has become a co-maintainer of the scheduler during the last year,
and Robert has become inactive - update the MAINTAINERS entry.

Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --cc MAINTAINERS
Simple merge