fix: update

Author: 0xPoe

statistics.md

@@ -752,7 +752,7 @@ The relationships of the relevant system variables are shown below:
 
 #### `tidb_build_stats_concurrency`
 
-Used to control the concurrency of table or partition analysis — for example, how many partition or table tasks can be processed simultaneously. The default value is `2`.
+Used to control the concurrency of table or partition analysis — for example, how many partition or table tasks can be processed simultaneously. The default value is `2`. The default value is `4` for v7.4.0 and earlier versions.
 
 #### `tidb_build_sampling_stats_concurrency`