docs(Readme): Fixes "Authentication Guide" link (#4113)
docs(Readme): Fixes "Authentication Guide" link (#4113)

* Fixes broken AUthentication Guide link in Readme(reoccurence of #3897)

* docs: Fixes broken "Authentication Guide" link in Readme(reoccurrence of #3897)

Co-authored-by: saranshdhingra <04ada9c25f3ab128cdd0f3112f842274dbc8154e@google.com>
chingor13/google-cloud-php,chingor13/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php,chingor13/google-cloud-php