lnrpc: Fix bug in 'cltv below minimum' error msg

Previously the error message produced when `CltvExpiry` is less
than the minimum final cltv (18 at present) set by
`routing.MinCLTVDelta` inserted the values into the wrong spots of the
formatted string.

lnrpc/invoicesrpc/addinvoice.go

@@ -367,7 +367,7 @@ func AddInvoice(ctx context.Context, cfg *AddInvoiceConfig,
 		if invoice.CltvExpiry < routing.MinCLTVDelta {
 			return nil, nil, fmt.Errorf("CLTV delta of %v must be "+
 				"greater than minimum of %v",
-				routing.MinCLTVDelta, invoice.CltvExpiry)
+				invoice.CltvExpiry, routing.MinCLTVDelta)
 		}
 
 		options = append(options,