fix error formatting (#3639)

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/transfer/setup.go

@@ -94,7 +94,7 @@ func parse(c *caddy.Controller) (*Transfer, error) {
 			}
 		}
 		if len(x.to) == 0 {
-			return nil, plugin.Error("transfer", c.Errf("'to' is required", c.Val()))
+			return nil, plugin.Error("transfer", c.Err("'to' is required"))
 		}
 		t.xfrs = append(t.xfrs, x)
 	}