tracing: fix document references

When moving documents to Documentation/trace/, I forgot to
grep Kconfig to find out those references.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Pekka Paalanen <pq@iki.fi>
Cc: eduard.munteanu@linux360.ro
LKML-Reference: <49DE97EF.7080208@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/Kconfig

@@ -312,7 +312,7 @@ config KMEMTRACE
 	  and profile kernel code.
 
 	  This requires an userspace application to use. See
-	  Documentation/vm/kmemtrace.txt for more information.
+	  Documentation/trace/kmemtrace.txt for more information.
 
 	  Saying Y will make the kernel somewhat larger and slower. However,
 	  if you disable kmemtrace at run-time or boot-time, the performance
@@ -403,7 +403,7 @@ config MMIOTRACE
 	  implementation and works via page faults. Tracing is disabled by
 	  default and can be enabled at run-time.
 
-	  See Documentation/tracers/mmiotrace.txt.
+	  See Documentation/trace/mmiotrace.txt.
 	  If you are not helping to develop drivers, say N.
 
 config MMIOTRACE_TEST