docs: trace: fix some Sphinx warnings

There are some warnings produced when building trace. Fix them.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.rst

@@ -1404,6 +1404,7 @@ trace has provided some very helpful debugging information.
 
 If we prefer function graph output instead of function, we can set
 display-graph option::
+
  with echo 1 > options/display-graph
 
   # tracer: irqsoff