docs: Fix broken link to generate anon and service keys in README.md

[nikiburggraf]

What kind of change does this PR introduce?

Docs update

What is the current behavior?

Docs previously pointed to this link, which doesn't point to what it says it does: https://supabase.com/docs/guides/self-hosting#api-keys

What is the new behavior?

Docs now point to this link, which contains the anon and secret key generator mentioned in the text around this: https://supabase.com/docs/guides/self-hosting/docker#generate-api-keys

Additional context

N/A