tracing: Fix incorrect file name

There is no file named 'enabled' in the directory tracing/events. It should
be the file 'enable'.

Signed-off-by: Xiongwei Song <sxwjean@me.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/events.txt

@@ -294,7 +294,7 @@ PID listed in the set_event_pid file.
 
 # cd /sys/kernel/debug/tracing
 # echo $$ > set_event_pid
-# echo 1 > events/enabled
+# echo 1 > events/enable
 
 Will only trace events for the current task.