trivial: ftrace:fix description of trace directory

Fix trace source directory from kernel/tracing/ to kernel/trace/.
Signed-off-by: GeunSik Lim <geunsik.lim@samsung.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/trace/ftrace.txt

@@ -1834,4 +1834,4 @@ an error.
 -----------
 
 More details can be found in the source code, in the
-kernel/tracing/*.c files.
+kernel/trace/*.c files.