tracing/doc: Fix ascii-art in histogram-design.rst

tracing/doc: Fix ascii-art in histogram-design.rst

This fixes the Sphinx parallel build error when building htmldocs:

  docutils.utils.SystemMessage: /home/sfr/next/next/Documentation/trace/histogram-design.rst:219: (SEVERE/4) Unexpected section title.

It also fixes a bunch of other warnings I noticed when fixing the
above, caused by mixing ascii-art and text.

Link: https://lkml.kernel.org/r/69c291c76964642a417e5dd170d183ba6b552010.camel@kernel.org
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>