Fix broken link in DM best practices (pingcap#18918)

dm/dm-best-practices.md

@@ -99,7 +99,7 @@ The following table summarizes the pros and cons of optimistic mode and pessimis
 
 | Scenario | Pros | Cons |
 | :--- | :--- | :--- |
-| Pessimistic mode (Default) | It can ensure that the data migrated to the downstream will not go wrong.  | If there are a large number of shards, the migration task will be blocked for a long time, or even stop if the upstream binlogs have been cleaned up. You can enable the relay log to avoid this problem. For more information, see [Use the relay log](#use-the relay-log). |
+| Pessimistic mode (Default) | It can ensure that the data migrated to the downstream will not go wrong.  | If there are a large number of shards, the migration task will be blocked for a long time, or even stop if the upstream binlogs have been cleaned up. You can enable the relay log to avoid this problem. For more information, see [Use the relay log](#use-the-relay-log). |
 | Optimistic mode| Upstream schema changes will not cause data migration latency.  | In this mode, ensure that schema changes are compatible (check whether the incremental column has a default value). It is possible that the inconsistent data can be overlooked. For more information, see [Merge and Migrate Data from Sharded Tables in Optimistic Mode](/dm/feature-shard-merge-optimistic.md#restrictions).|
 
 ### Other restrictions and impact