qapi: Fix document for BlockStats.node-name

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1418694024-26498-1-git-send-email-famz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>