docs: fix broken reference

docs: fix broken reference

Long story short, we need a space here for the reference to work
correctly.

Longer story:

Without the space, kerneldoc generates a line like this:

one of :c:type:`MemoryListener.region_add\(\) <MemoryListener>`,:c:type:`MemoryListener.region_del\(\)

Sphinx does not process the role information correctly, so we get this
(my pseudo-notation) construct:

<text>,:c:type:</text>
<reference target="MemoryListener">MemoryListener.region_del()</reference>

which does not reference the desired entity, and leaves some extra junk
in the rendered output. See
https://qemu-project.gitlab.io/qemu/devel/memory.html#c.MemoryListener
member log_start for an example of the broken output as it looks today.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210511192950.2061326-1-jsnow@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>