Documentation: trace: historgram-design: Separate sched_waking histogram section heading and the following diagram

Section heading for sched_waking histogram is shown as normal paragraph
instead due to codeblock marker for the following diagram being in the
same line as the section underline. Separate them.

Fixes: daceabf ("tracing/doc: Fix ascii-art in histogram-design.rst")
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250916054202.582074-5-bagasdotme@gmail.com>

Author: bagasme

Documentation/trace/histogram-design.rst

@@ -380,7 +380,9 @@ entry, ts0, corresponding to the ts0 variable in the sched_waking
 trigger above.
 
 sched_waking histogram
-----------------------::
+----------------------
+
+.. code-block::
 
   +------------------+
   | hist_data        |<-------------------------------------------------------+