Documentation: kprobetrace: Fix code block markup

This display the following code extract as a code block instead of a
normal paragraph.

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20230121225304.1711635-3-yoann.congal@smile.fr


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