doc: tweak dm-parameter doc
Remove trailing blanks, and change one last reviewer comment.
@@ -275,7 +275,7 @@ Here are descriptions for each of these ``acrn-dm`` command line parameters:
* - :kbd:`--virtio_poll <poll_interval>`
- Enable virtio poll mode with poll interval xxx ns.
- usage::
+ Example::
--virtio_poll 1000000
