docs: ftrace: fix the wrong path of tracefs

Delete "tracing" due to it has been included in /proc/mounts.
Delete "echo nop > $tracefs/tracing/current_tracer", maybe
this command is redundant.

Signed-off-by: Zhaoyu Liu <zackary.liu.pro@gmail.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>