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The file tracing_enable is obsolete and does not exist anymore.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>