update readme

Author: jdpedrie

cloud_sql/sqlserver/pdo/README.md

@@ -15,7 +15,7 @@ export CLOUD_SQL_CONNECTION_NAME='<MY-PROJECT>:<INSTANCE-REGION>:<MY-DATABASE>'
 export DB_USER='my-db-user'
 export DB_PASS='my-db-pass'
 export DB_NAME='my-db-name'
-export DB_HOSTNAME='localhost'
+export DB_HOSTNAME='(local)'
 ```
 
 Note: Saving credentials in environment variables is convenient, but not secure - consider a more secure solution such as [Cloud KMS](https://cloud.google.com/kms/) to help keep secrets safe.