tracing: Fix warnings when building htmldocs for function graph retval

When building htmldocs, the following warnings appear:

Documentation/trace/ftrace.rst:2797: WARNING: Literal block expected; none found.
Documentation/trace/ftrace.rst:2816: WARNING: Literal block expected; none found.

So fix it.

Link: https://lore.kernel.org/all/20230623143517.19ffc6c0@canb.auug.org.au/
Link: https://lkml.kernel.org/r/20230623071728.25688-1-pengdonglin@sangfor.com.cn

Fixes: 21c094d3

 ("tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex")
Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>