Update instructions for Google ID token
Update instructions for Google ID token

Update instructions to configure backend for Google ID token authentication.

Signed-off-by: Christophe Taton <a5b67bfcd23a92efc73996e448d7580cf8cc69a8@gmail.com>

GoogleCloudPlatform/endpoints-samples,GoogleCloudPlatform/endpoints-samples,GoogleCloudPlatform/endpoints-samples,GoogleCloudPlatform/endpoints-samples,GoogleCloudPlatform/endpoints-samples,GoogleCloudPlatform/endpoints-samples