vhost: Fix documentation

Fix documentation to match actual function prototypes

"end" used instead of "last". Fix that.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Link: https://lore.kernel.org/r/20200630052925.GA157062@mtl-vdi-166.wap.labs.mlnx


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>