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

Documentation/ftrace.txt

@@ -100,7 +100,7 @@ of ftrace. Here is a list of some of the key files:
 	that is displayed in one of the above output
 	files.
 
-  trace_max_latency:
+  tracing_max_latency:
 
 	Some of the tracers record the max latency.
 	For example, the time interrupts are disabled.