docs: tracing: Fix a broken label

docs: tracing: Fix a broken label

Sphinx warnings about his:

	Documentation/trace/kprobetrace.rst:68: WARNING: undefined label: user_mem_access (if the link has no caption the label must precede a section header)

The problem is quite simple: Sphinx wants a blank line after
references.

Link: http://lkml.kernel.org/r/a83ea390bc28784518fce772b4c961ea1c976f14.1560883872.git.mchehab+samsung@kernel.org
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>