Fixed #26121 -- Updated MySQL storage engine example.

default_storage_engine was introduced in MySQL 5.5.3.
storage_engine was removed in MySQL 5.7.5.

docs/ref/databases.txt

@@ -461,7 +461,7 @@ storage engine, you have a couple of options.
   creating your tables::
 
       'OPTIONS': {
-         'init_command': 'SET storage_engine=INNODB',
+         'init_command': 'SET default_storage_engine=INNODB',
       }
 
   This sets the default storage engine upon connecting to the database.