docs: ftrace: fix a issue with duplicated subtitle number

The subtitle "5.3 Clearing filters" and "5.3 Subsystem filters" has
the same index number, let's fix it.

Fixes: 95b696088c1c ("tracing/filters: add filter Documentation")
Signed-off-by: Donglin Peng <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/trace/events.rst

···
 To clear the filters for all events in a subsystem, write a '0' to the
 subsystem's filter file.
 
-5.3 Subsystem filters
+5.4 Subsystem filters
 ---------------------
 
 For convenience, filters for every event in a subsystem can be set or
···
 	# cat sched_wakeup/filter
 	common_pid == 0
 
-5.4 PID filtering
+5.5 PID filtering
 -----------------
 
 The set_event_pid file in the same directory as the top events directory