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>