Update src/useful/domain-structure.md

Co-authored-by: JustDeveloper <[email protected]>

Author: wdhdev

src/useful/domain-structure.md

@@ -215,7 +215,7 @@ Enable Cloudflare proxy for your domain. Disabled by default. To enable it, add
 "proxied": true
 ```
 
-### redirect_config
+### redirect_config (*optional*)
 - Allows custom redirect endpoints for your domain. An example can be found [here](https://github.com/is-a-dev/register/blob/main/domains/william.json).
 ```json
 "redirect_config": {