filesender: FIX: missing format definition.

Seems like there is missing format definition in example.

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

nixos/modules/services/web-apps/filesender.md

@@ -9,6 +9,9 @@ FileSender uses [SimpleSAMLphp](https://simplesamlphp.org/) for authentication,
 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 = {