docs: fix drop stream syntax (#16550)

docs/en/12-taos-sql/14-stream.md

@@ -50,7 +50,7 @@ SELECT _wstartts, count(*), avg(voltage) FROM meters PARTITION BY tbname INTERVA
 ## Delete a Stream
 
 ```sql
-DROP STREAM [IF NOT EXISTS] stream_name
+DROP STREAM [IF EXISTS] stream_name
 ```
 
 This statement deletes the stream processing service only. The data generated by the stream is retained.

docs/zh/12-taos-sql/14-stream.md

@@ -58,7 +58,7 @@ SELECT _wstartts, count(*), avg(voltage) FROM meters PARTITION BY tbname INTERVA
 ## 删除流式计算
 
 ```sql
-DROP STREAM [IF NOT EXISTS] stream_name;
+DROP STREAM [IF EXISTS] stream_name;
 ```
 
 仅删除流式计算任务，由流式计算写入的数据不会被删除。