ftrace: fix documentation typo s/trace_max_latency/tracing_max_latency/

There isn't a trace_max_latency file, there is tracing_max_latency.
Fix it.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Steven Rostedt <srostedt@redhat.com>
LKML-Reference: <20090307235409.5A87.A69D9226@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>