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: 95b69608 ("tracing/filters: add filter Documentation")
Signed-off-by: Donglin Peng <dolinux.peng@gmail.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20221209025119.1371570-1-dolinux.peng@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>