README: replace tx_isolation with transaction_isolation (#1077)

Author: nozawana44

README.md

@@ -376,7 +376,7 @@ Rules:
 Examples:
   * `autocommit=1`: `SET autocommit=1`
   * [`time_zone=%27Europe%2FParis%27`](https://dev.mysql.com/doc/refman/5.5/en/time-zone-support.html): `SET time_zone='Europe/Paris'`
-  * [`tx_isolation=%27REPEATABLE-READ%27`](https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_tx_isolation): `SET tx_isolation='REPEATABLE-READ'`
+  * [`transaction_isolation=%27REPEATABLE-READ%27`](https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_transaction_isolation): `SET transaction_isolation='REPEATABLE-READ'`
 
 
 #### Examples