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>

Documentation/trace/ftrace.rst

@@ -2442,11 +2442,10 @@ Or this simple script!
   #!/bin/bash
 
   tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
-  echo nop > $tracefs/tracing/current_tracer
-  echo 0 > $tracefs/tracing/tracing_on
-  echo $$ > $tracefs/tracing/set_ftrace_pid
-  echo function > $tracefs/tracing/current_tracer
-  echo 1 > $tracefs/tracing/tracing_on
+  echo 0 > $tracefs/tracing_on
+  echo $$ > $tracefs/set_ftrace_pid
+  echo function > $tracefs/current_tracer
+  echo 1 > $tracefs/tracing_on
   exec "$@"