selftests/ftrace: Correctly enable event in instance-event.tc

Function instance_set() expects to enable event 'sched_switch', so we
should set 1 to its 'enable' file.

Testcase passed after this patch:
  # ./ftracetest test.d/instances/instance-event.tc
  === Ftrace unit tests ===
  [1] Test creation and deletion of trace instances while setting an event
  [PASS]

  # of passed:  1
  # of failed:  0
  # of unresolved:  0
  # of untested:  0
  # of unsupported:  0
  # of xfailed:  0
  # of undefined(test bug):  0
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/ftrace/test.d/instances/instance-event.tc

@@ -39,7 +39,7 @@ instance_read() {
 
 instance_set() {
         while :; do
-                echo 1 > foo/events/sched/sched_switch
+                echo 1 > foo/events/sched/sched_switch/enable
         done 2> /dev/null
 }