Documentation: Fix the wrong command `echo -1 > set_ftrace_pid` for cleaning the filter.

The command `echo > set_ftrace_pid` should be used to clean the filter quietly.
because the command `echo -1 >  set_ftrace_pid` will output the following:
"bash: echo: write error: Invalid argument".

so update the file Documentation/trace/ftrace.txt.
Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.txt

@@ -1740,7 +1740,7 @@ no pid
      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
-# echo -1 > set_ftrace_pid
+# echo > set_ftrace_pid
 # cat trace |head
  # tracer: function
  #