Update tidb-distributed-execution-framework.md

tidb-distributed-execution-framework.md

@@ -59,6 +59,7 @@ Currently, for TiDB Self-Hosted, the DXF supports the distributed execution of t
 
 - The DXF can only schedule the distributed execution of one `ADD INDEX` task at a time. If a new `ADD INDEX` task is submitted before the current `ADD INDEX` distributed task has finished, the new task is executed through a transaction.
 - Adding indexes on columns with the `TIMESTAMP` data type through the DXF is not supported, because it might lead to inconsistency between the index and the data.
+
 ## Prerequisites
 
 Before using the distributed framework, you need to enable the [Fast Online DDL](/system-variables.md#tidb_ddl_enable_fast_reorg-new-in-v630) mode.