Correct idempotency key docs on LSPS2ServiceEvent::OpenChannel

While it would be correct to use `(their_network_key,
intercept_scid)` as the idempotency key when handling
`LSPS2ServiceEvent::OpenChannel`, we don't actually expect anyone
to do so as it would require separate storage to track the
`intercept_scid` -> opened channel mappings.

Thus, we update the documentation to note that the correct
idempotency key is `(their_network_key, user_channel_id)`.

Author: TheBlueMatt

lightning-liquidity/src/lsps2/event.rs

@@ -163,7 +163,7 @@ pub enum LSPS2ServiceEvent {
 	///
 	/// **Note: ** As this event is persisted and might get replayed after restart, you'll need to
 	/// ensure channel creation idempotency. I.e., please check if you already created a
-	/// corresponding channel based on the given `their_network_key` and `intercept_scid` and
+	/// corresponding channel based on the given `their_network_key` and `user_channel_id` and
 	/// ignore this event in case you did.
 	///
 	/// [`ChannelManager::create_channel`]: lightning::ln::channelmanager::ChannelManager::create_channel