EXAMPLE: fix with-supabase signup button style (#53531)

### What?
Fix example `with-supabase` https://github.com/vercel/next.js/tree/canary/examples/with-supabase signup button

### Why?
Signup button text color is not the right one so the developer is not able to see the text.

### How?
Updated the style using tailwind as it is what we are using in the example.

Before:
![image](https://github.com/vercel/next.js/assets/1652286/269c3be1-70ee-4b92-9161-4d1c35cbd01c)

After (Fixed):
![image](https://github.com/vercel/next.js/assets/1652286/3aa6af87-7958-470d-bf05-d1187963e792)

examples/with-supabase/app/login/page.tsx

@@ -54,7 +54,7 @@ export default function Login() {
         </button>
         <button
           formAction="/auth/sign-up"
-          className="border border-gray-700 rounded px-4 py-2 text-white mb-2"
+          className="border border-gray-700 rounded px-4 py-2 text-black mb-2"
         >
           Sign Up
         </button>