debugfs: Fix mount directory of debugfs by default in events.txt

We need common default directory of denbugfs for consistency.
(debugfs's default directory is /sys/kernel/debug/ by debugfs.)

Signed-off-by: GeunSik Lim <geunsik.lim@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>