docs: trace: add a missing blank line

Sphinx expects a blank line after a literal block markup.
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: NMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/trace/kprobetrace.rst

@@ -191,6 +191,7 @@ events, you need to enable it.
 
 Use the following command to start tracing in an interval.
 ::
+
     # echo 1 > tracing_on
     Open something...
     # echo 0 > tracing_on