virtio_ring: Fix kernel-doc

Fix function name in virtio_ring.c kernel-doc comment
to remove a warning found by clang_w1.

drivers/virtio/virtio_ring.c:1903: warning: expecting prototype for
virtqueue_get_buf(). Prototype was for virtqueue_get_buf_ctx() instead

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/1621998731-17445-1-git-send-email-yang.lee@linux.alibaba.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>