fix sample gae definition

Author: jdpedrie

cloud_sql/postgres/pdo/app-standard.yaml

@@ -17,7 +17,7 @@ runtime: php72
 # Remember - storing secrets in plaintext is potentially unsafe. Consider using
 # something like https://cloud.google.com/kms/ to help keep secrets secret.
 env_variables:
-  CLOUD_SQL_CONNECTION_NAME: ::<MY-DATABASE>
+  CLOUD_SQL_CONNECTION_NAME: ::<INSTANCE-NAME>
   DB_USER: my-db-user
   DB_PASS: my-db-pass
   DB_NAME: my-db