Update custom domain example

Author: NhienLam

auth-next/email-link-auth.js

@@ -20,7 +20,7 @@ function emailLinkActionCodeSettings() {
       minimumVersion: '12'
     },
     // The domain must be configured in Firebase Hosting and owned by the project.
-    linkDomain: 'example.com'
+    linkDomain: 'custom-domain.com'
   };
   // [END auth_email_link_actioncode_settings]
 }

snippets/auth-next/email-link-auth/auth_email_link_actioncode_settings.js

@@ -20,6 +20,6 @@ const actionCodeSettings = {
     minimumVersion: '12'
   },
   // The domain must be configured in Firebase Hosting and owned by the project.
-  linkDomain: 'example.com'
+  linkDomain: 'custom-domain.com'
 };
 // [END auth_email_link_actioncode_settings_modular]