Event name for hot unplug errors was wrong.
Make doc match code.

Cc: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>