fix: remove iframe description (#654)

Author: wangsijie

docs/docs/recipes/custom-domain/README.mdx

@@ -15,7 +15,7 @@ You can use your own domain name for your Logto tenant. This feature lets you pr
 By using your own domain name for your Logto tenant, you'll benefit from several advantages:
 
 - Branding and Trust: Your users will see your domain name in the browser's address bar when they sign in to your application. By avoiding redirects to a third-party domain site, your users are provided with a sense of security and trust.
-- Versatility: Having a sub-domain can be beneficial if you wish to use your own wildcard SSL certificate, cross-domain cookies, or if you need to utilize iframes.
+- Versatility: Having a sub-domain can be beneficial if you wish to use your own wildcard SSL certificate, or cross-domain cookies.
 
 ## How Does it Work?