ping: First parameter is called 'id' not 'peerid', fix help

The first argument of 'ping' was documented as 'peerid', however
internally it is expected to be just 'id'.

To avoid breaking the API, opt to fix the documentation.

lightningd/ping.c

@@ -86,6 +86,6 @@ static void json_ping(struct command *cmd,
 static const struct json_command ping_command = {
 	"ping",
 	json_ping,
-	"Send {peerid} a ping of length {len} (default 128) asking for {pongbytes} (default 128)"
+	"Send peer {id} a ping of length {len} (default 128) asking for {pongbytes} (default 128)"
 };
 AUTODATA(json_command, &ping_command);