sched/debug: Update stale reference to sched_debug.c

Since commit:

   8a99b683 ("sched: Move SCHED_DEBUG sysctl to debugfs")

The sched_debug interface moved from /proc to debugfs. The comment
mentions still the outdated proc interfaces.

Update the comment, point to the current location of the interface.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230920130025.412071-3-bigeasy@linutronix.de