onlyoffice: fix syntax error

The `more_set_headers` syntax was wrong.

Syntax docs: https://github.com/openresty/headers-more-nginx-module?tab=readme-ov-file#more_set_headers

This led to:

```
{\"Exception\":\"InvalidArgumentException\",\"Message\":\"\\\"attachment;\\\" is not valid header name.\",\"Code\":0,\"Trace\":[
{\"file\":\"/nix/store/4y4c191pd9pa5nd78i8flvkf50gd4lvp-nextcloud-31.0.8/3rdparty/guzzlehttp/psr7/src/MessageTrait.php\",\"line\":153,\"function\":\"assertHeader\",\"class\":\"GuzzleHttp\\\\Psr7\\\\Response\",\"type\":\"->\"},
```

Introduced in https://github.com/NixOS/nixpkgs/pull/419765

Changed files
+1 -1
nixos
modules
services
web-apps
+1 -1
nixos/modules/services/web-apps/onlyoffice.nix
···
'';
"~* ^(\\/cache\\/files.*)(\\/.*)".extraConfig = ''
alias /var/lib/onlyoffice/documentserver/App_Data$1;
-
more_set_headers Content-Disposition "attachment; filename*=UTF-8''$arg_filename";
+
more_set_headers "Content-Disposition: attachment; filename*=UTF-8''$arg_filename";
set $secure_link_secret verysecretstring;
secure_link $arg_md5,$arg_expires;