doc: fix typo in vhost document

In "Example output"
'./scripts/rpc.py bdev_malloc_create 128 4096 Malloc0'
should be
'./scripts/rpc.py bdev_malloc_create 128 4096 -b Malloc0'
Add the missing '-b'.

Signed-off-by: Roger <lichangxinlivingshade@gmail.com>
Change-Id: I81b7eb122d8a4f115ec23c9e017b235d36b2d7d1
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/15531


Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>