doc: fix old config name of kprobetrace

KPROBE_TRACING has been replaced by KPROBE_EVENT.

Signed-off-by: Liu Bo <liub.liubo@gmail.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>