docs: Fix dimm example

In the example for <memory model='dimm'/> we show how to
configure hugepages as backend. In the example we show 4MiB
hugepages which are non-standard and thus at the first glance may
mislead users thinking that a regular sized pages (4K) will be
used. Use 2MiB as the value instead.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

docs/formatdomain.rst

@@ -7745,7 +7745,7 @@ Example: usage of the memory devices
      </memory>
      <memory model='dimm'>
        <source>
-         <pagesize unit='KiB'>4096</pagesize>
+         <pagesize unit='KiB'>2048</pagesize>
          <nodemask>1-3</nodemask>
        </source>
        <target>