invoice: fix up internal help msg to match updated expiry default.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/invoice.c

@@ -792,7 +792,7 @@ static const struct json_command invoice_command = {
 	json_invoice,
 	"Create an invoice for {msatoshi} with {label} "
 	"and {description} with optional {expiry} seconds "
-	"(default 1 hour), optional {fallbacks} address list"
+	"(default 1 week), optional {fallbacks} address list"
 	"(default empty list) and optional {preimage} "
 	"(default autogenerated)"};
 AUTODATA(json_command, &invoice_command);