document Auth0 example on auth

**Describe the content issue**:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

https://github.com/aws-amplify/amplify-backend/issues/1526

**URL page where content issue is**:
https://docs.amplify.aws/react/build-a-backend/auth/concepts/external-identity-providers/