[docs] fix(with-supabase): Update README env variables (#84745)
Closes #84734
Fixes environmental variable names in `with-supabase` example README to
match the actual `.env.example` file.
Updates `NEXT_PUBLIC_SUPABASE_ANON_KEY` to
`NEXT_PUBLIC_SUPABASE_PUBLISHABLE_OR_ANON_KEY` in two locations.
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
