samples: fix the prompt about SAMPLE_VFIO_MDEV_MBOCHS

Change the prompt about SAMPLE_VFIO_MDEV_MBOCHS as
'Build VFIO mbochs example mediated device sample code'.

Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Link: https://lore.kernel.org/r/202301301013518438986@zte.com.cn


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>