Fix Cloud SQL example (MySQL) (#2309)

The sample has the wrong format for `unix_socket` inside the query parameter in sqlalchemy.create_engine.

Doing:

    '/cloudsql/{}/'.format(cloud_sql_connection_name)

takes the path as a directory so the ending slash needs to be removed:

    '/cloudsql/{}'.format(cloud_sql_connection_name)

Author: joangro

cloud-sql/mysql/sqlalchemy/main.py

@@ -43,7 +43,7 @@
         password=db_pass,
         database=db_name,
         query={
-            'unix_socket': '/cloudsql/{}/'.format(cloud_sql_connection_name)
+            'unix_socket': '/cloudsql/{}'.format(cloud_sql_connection_name)
         }
     ),
     # ... Specify additional properties here.