Fix a VDUSE documentation build warning:

Documentation/userspace-api/vduse.rst:21: WARNING: Title underline too short.

Fixes: 7bc7f618 ("Documentation: Add documentation for VDUSE")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Xie Yongji <xieyongji@bytedance.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20211006202904.30241-1-rdunlap@infradead.orgSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>