Update system-variables.md

Co-authored-by: Lynn <zimu_xia@126.com>

system-variables.md

@@ -1152,7 +1152,7 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 - Persists to cluster: Yes
 - Default value: `OFF`
 - This variable is used to control whether to enable distributed execution of the DDL reorg phase to improve the speed of this phase. Currently, this variable is only valid for the `ADD INDEX` statement. Enabling this variable improves the performance of large tables. Distributed DDL execution can control the CPU usage of DDL through dynamic DDL resource management to prevent DDL from affecting the online application.
-- To verify whether a completed `ADD INDEX` operation is accelerated by this feature, you can check whether a corresponding task is in the `mysql.tidb_ddl_backfill_history` table.
+- To verify whether a completed `ADD INDEX` operation is accelerated by this feature, you can check whether a corresponding task is in the `mysql.tidb_background_subtask_history` table.
 
 ### tidb_ddl_error_count_limit