fix: slug of custom domains howto

Author: asaf

docs/concepts/custom_domain.md

@@ -19,3 +19,7 @@ This has several advantages:
 - Branding, your users don't leave your own domain.
 - Easy migration between identity services.
 - Security and cookie data sharing.
+
+:::info
+For instructions, see [Custom Domains How-To](/docs/guides/howto/custom-domain)
+:::

docs/guides/howto/custom_domain.md

@@ -2,7 +2,7 @@
 id: custom_domain
 title: Add custom domain.
 sidebar_label: Custom Domain
-slug: /guides/howto/custom_domain
+slug: /guides/howto/custom-domain
 description: How to add a custom domain.
 ---