filesender: FIX: missing format definition.

Seems like there is missing format definition in example.

Affected line:
default = format.lib.mkMixedArray [ "exampleauth:UserPass" ] {

Changed files
+3
nixos
modules
services
web-apps
+3
nixos/modules/services/web-apps/filesender.md
···
Minimal working instance of FileSender that uses password-authentication would look like this:
```nix
+
let
+
format = pkgs.formats.php {};
+
in
{
networking.firewall.allowedTCPPorts = [ 80 443 ];
services.filesender = {