Update placement-rules-in-sql.md

placement-rules-in-sql.md

@@ -18,7 +18,7 @@ The detailed user scenarios are as follows:
 - Store new data into NVMe storage and store old data into SSDs to lower the cost on data archiving and storage 
 - Schedule the leaders of hotspot data to high-performance TiKV instances
 - Separate cold data to lower-cost storage mediums to improve cost efficiency
-- Support the physical isolation of computing resources between different users, which meets the isolation requirements of different users within an instance, and the isolation requirements of CPU, I/O, memory, and other resources with different mixed loads
+- Support the physical isolation of computing resources between different users, which meets the isolation requirements of different users in a cluster, and the isolation requirements of CPU, I/O, memory, and other resources with different mixed loads
 
 ## Specify placement rules