docs: Fix nvdimm example wrt to <uuid/>

docs: Fix nvdimm example wrt to <uuid/>

On PPC platform it is required that a NVDIMM has an UUID. If none
is provided then libvirt generates one during parsing (see
v6.2.0-rc1~96 and friends). However, the example provided in our
documentation is not valid XML.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Han Han <hhan@redhat.com>