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>