tracing: Fix comment to use tracing_on over tracing_enable

The file tracing_enable is obsolete and does not exist anymore. Replace
the comment that references it with the proper tracing_on file.

Link: http://lkml.kernel.org/r/1450787141-45544-1-git-send-email-chuhu@redhat.com

Signed-off-by: Chuyu Hu <chuhu@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -363,8 +363,8 @@ struct trace_option_dentry {
  * @name: the name chosen to select it on the available_tracers file
  * @init: called when one switches to this tracer (echo name > current_tracer)
  * @reset: called when one switches to another tracer
- * @start: called when tracing is unpaused (echo 1 > tracing_enabled)
- * @stop: called when tracing is paused (echo 0 > tracing_enabled)
+ * @start: called when tracing is unpaused (echo 1 > tracing_on)
+ * @stop: called when tracing is paused (echo 0 > tracing_on)
  * @update_thresh: called when tracing_thresh is updated
  * @open: called when the trace file is opened
  * @pipe_open: called when the trace_pipe file is opened