nixos/onlyoffice: update maintainership

Sandro expressed desire to transfer maintainership of this package.
While I don't expect to be a super active maintainer, I do use this
package and am relatively familiar with its internals, so I can take
this on.

nixos/modules/services/web-apps/onlyoffice.nix

@@ -343,4 +343,6 @@ in
 
     users.groups.onlyoffice = { };
   };
+
+  meta.maintainers = with lib.maintainers; [ raboof ];
 }