doc: Fix a documentation error in sendonion

Te `sendonion` docs where claiming that the `first_hop` needs to specify a
`channel_id` whereas it should really be the `node_id` of the peer we are
trying to contact.

doc/lightning-sendonion.7

@@ -31,13 +31,13 @@ further details\.
 The \fIfirst_hop\fR parameter instructs c-lightning which peer to send the onion
 to\. It is a JSON dictionary that corresponds to the first element of the route
 array returned by \fIgetroute\fR\. The following is a minimal example telling
-c-lightning to use channel \fB103x1x1\fR to add an HTLC for 1002 millisatoshis and
-a delay of 21 blocks on top of the current blockheight:
+c-lightning to use any available channel to \fB022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59\fR
+to add an HTLC for 1002 millisatoshis and a delay of 21 blocks on top of the current blockheight:
 
 .nf
 .RS
 {
-  "channel": "103x1x1",
+  "id": "022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59",
   "direction": 1,
   "amount_msat": "1002msat",
   "delay": 21,

doc/lightning-sendonion.7.md

@@ -30,12 +30,12 @@ further details.
 The *first_hop* parameter instructs c-lightning which peer to send the onion
 to. It is a JSON dictionary that corresponds to the first element of the route
 array returned by *getroute*. The following is a minimal example telling
-c-lightning to use channel `103x1x1` to add an HTLC for 1002 millisatoshis and
-a delay of 21 blocks on top of the current blockheight:
+c-lightning to use any available channel to `022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59`
+to add an HTLC for 1002 millisatoshis and a delay of 21 blocks on top of the current blockheight:
 
 ```json
 {
-  "channel": "103x1x1",
+  "id": "022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59",
   "direction": 1,
   "amount_msat": "1002msat",
   "delay": 21,