ticdc: add a note about schema (#18816) (#18819)

ticdc/ticdc-sink-to-kafka.md

@@ -225,10 +225,10 @@ dispatchers = [
 
 You can use topic = "xxx" to specify a Topic dispatcher and use topic expressions to implement flexible topic dispatching policies. It is recommended that the total number of topics be less than 1000.
 
-The format of the Topic expression is `[prefix][{schema}][middle][{table}][suffix]`.
+The format of the Topic expression is `[prefix]{schema}[middle][{table}][suffix]`.
 
 - `prefix`: optional. Indicates the prefix of the Topic Name.
-- `[{schema}]`: optional. Used to match the schema name.
+- `{schema}`: required. Used to match the schema name. Starting from v7.1.4, this parameter is optional.
 - `middle`: optional. Indicates the delimiter between schema name and table name.
 - `{table}`: optional. Used to match the table name.
 - `suffix`: optional. Indicates the suffix of the Topic Name.