nextjs.mdx: Correct links in nextjs auth example (#33465)

The docs directed to localhost:3000/login
It seems the nextjs template now has a new route `/sign-in`
Added note: 
the route does not become available until .env.local is in env

apps/docs/content/guides/auth/quickstarts/nextjs.mdx

@@ -72,7 +72,7 @@ hideToc: true
 
     Start the development server, go to http://localhost:3000 in a browser, and you should see the contents of `app/page.tsx`.
 
-    To sign up a new user, navigate to http://localhost:3000/login, and click `Sign Up Now`.
+    To sign up a new user, navigate to http://localhost:3000/sign-up, and click `Sign up`. *NOTE: .env.example must be renamed to .env.local before this route becomes available*
 
     </StepHikeCompact.Details>