Update shadcn-ui.mdx instructions (#2176)

Co-authored-by: Alexis Aguilar <[email protected]>

Author: cyupa

docs/customization/elements/examples/shadcn-ui.mdx

@@ -39,6 +39,7 @@ You must also configure the appropriate settings in Clerk:
 
 1. In the Clerk Dashboard, navigate to the [**SSO connections**](https://dashboard.clerk.com/last-active?path=user-authentication/sso-connections) page.
 1. Ensure that **Google** and **GitHub** are enabled. If they are not in the list of connections, select the **Add connection** button, and select **For all users**. Enable **Google** and **GitHub**.
+1. Make sure you have your sign in and sign up URLs set in your [environment variables](https://clerk.com/docs/guides/custom-redirects#environment-variables).
 
 ## Sign up