docs: incorrect transaction level in docs, v4 use db default (#8020)

docs/transactions.md

@@ -124,7 +124,7 @@ Sequelize.Transaction.ISOLATION_LEVELS.REPEATABLE_READ  // "REPEATABLE READ"
 Sequelize.Transaction.ISOLATION_LEVELS.SERIALIZABLE // "SERIALIZABLE"
 Sequelize.Transaction.ISOLATION_LEVELS.SERIALIZABLE // "SERIALIZABLE"
 ```
 ```
 
 
-By default, sequelize uses "REPEATABLE READ". If you want to use a different isolation level, pass in the desired level as the first argument:
+By default, sequelize uses the isolation level of the database. If you want to use a different isolation level, pass in the desired level as the first argument:
 
 
 ```js
 ```js
 return sequelize.transaction({
 return sequelize.transaction({