apache-httpd: fix typo in config servedFiles

nixos/modules/services/web-servers/apache-httpd/per-server-options.nix

···
     default = [];
     example = [
       { urlPath = "/foo/bar.png";
-        files = "/home/eelco/some-file.png";
+        file = "/home/eelco/some-file.png";
       }
     ];
     description = ''