examples(with-supabase): fix action type error (#72783)
## Why?
There is a type issue with one of the form actions.
```
app/(auth-pages)/sign-up/page.tsx:42:25 - error TS2322: Type '(formData: FormData) => Promise<{ error: string; }>' is not assignable to type 'string | ((formData: FormData) => void | Promise<void>) | undefined'.
Type '(formData: FormData) => Promise<{ error: string; }>' is not assignable to type '(formData: FormData) => void | Promise<void>'.
Type 'Promise<{ error: string; }>' is not assignable to type 'void | Promise<void>'.
Type 'Promise<{ error: string; }>' is not assignable to type 'Promise<void>'.
Type '{ error: string; }' is not assignable to type 'void'.
42 <SubmitButton formAction={signUpAction} pendingText="Signing up...">
```
- x-ref: https://github.com/vercel/next.js/discussions/72778
