Change <entity/> element to <subscription/> element in response to subscription request

We missed one of the latest changes in v1.8.

Example 28. Entity subscribes to a node

<iq type='set'
from='francisco@denmark.lit/barracks'
to='pubsub.shakespeare.lit'
id='sub1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscribe
node='princely_musings'
jid='francisco@denmark.lit'/>
</pubsub>
</iq>

If the subscription request is successfully processed, the server MUST inform the requesting entity that it is now subscribed (which MAY include a service-generated SubID).

Example 29. Service replies with success

<iq type='result'
from='pubsub.shakespeare.lit'
to='francisco@denmark.lit/barracks'
id='sub1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscription
node='princely_musings'
jid='francisco@denmark.lit'
subid='ba49252aaa4f5d320c24d3766f0bdcade78c78d3'
subscription='subscribed'/>
</pubsub>
</iq>