s/types.string/types.str/

`types.string` was being used for too many thing so it got deprecated
and now there are several different string types. `types.str` is the one
where you don't want to merge definitions if it's defined in more than
one place

Co-authored-by: Taeer Bar-Yam <taeer.bar-yam@moduscreate.com>

fediversity/default.nix

@@ -22,7 +22,7 @@ in {
       };
 
       domain = mkOption {
-        type = types.string;
+        type = types.str;
       };
 
       mastodon.enable = mkEnableOption "default Fediversity Mastodon configuration";