Documentation/trace/events.txt: Remove obsolete sched_signal_send.

'sched_signal_send' doesn't exist in the kernel now. Replace it
with 'signal_generate' so that the example in events.txt can be
executed successfully.

I also change some /debug to /sys/kernel/debug which is debugfs'
default mount directory now(similar to commit 52ad51e7).

Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>