ReferenceError in authentication.md example fixed

[hanneslund]

Before

export const getServerSideProps = withSession(async function ({ req, res }) {
  if (!req.session.user) {
    return {
      redirect: {
        destination: '/login',
        permanent: false,
      },
    }
  }

  return {
    props: { user }, // User not defined
  }
})

[leerob]

Thank you!