sched: fix RLIMIT_CPU comment

Devan Lippman noticed that the RLIMIT_CPU comment in resource.h is
incorrect: the field is in seconds, not msecs. We used msecs in
earlier versions of the patch but that got changed.

Found-by: Devan Lippman <devan.lippman@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>