Update email link auth snippet to stop using deprecated dynamicLinkDomain (#390)

* Update email link auth snippet to remove FDL

* Update custom domain example

Author: NhienLam

auth-next/email-link-auth.js

@@ -19,7 +19,8 @@ function emailLinkActionCodeSettings() {
       installApp: true,
       minimumVersion: '12'
     },
-    dynamicLinkDomain: 'example.page.link'
+    // The domain must be configured in Firebase Hosting and owned by the project.
+    linkDomain: 'custom-domain.com'
   };
   // [END auth_email_link_actioncode_settings]
 }

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

@@ -19,6 +19,7 @@ const actionCodeSettings = {
     installApp: true,
     minimumVersion: '12'
   },
-  dynamicLinkDomain: 'example.page.link'
+  // The domain must be configured in Firebase Hosting and owned by the project.
+  linkDomain: 'custom-domain.com'
 };
 // [END auth_email_link_actioncode_settings_modular]