DOC: update README

Signed-off-by: George Chen <[email protected]>

data-prepper-plugins/opensearch/README.md

@@ -133,7 +133,7 @@ Default is null.
   * This index name can also be a plain string plus a date-time pattern as a suffix, such as `application-%{yyyy.MM.dd}`, `my-index-name-%{yyyy.MM.dd.HH}`. When OpenSearch Sink is sending data to OpenSearch, the date-time pattern will be replaced by actual UTC time. The pattern supports all the symbols that represent one hour or above and are listed in [Java DateTimeFormatter](https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html). For example, with an index pattern like `my-index-name-%{yyyy.MM.dd}`, a new index is created for each day such as `my-index-name-2022.01.25`. For another example, with an index pattern like `my-index-name-%{yyyy.MM.dd.HH}`, a new index is created for each hour such as `my-index-name-2022.01.25.13`.
   * This index name can also be a formatted string (with or without date-time pattern suffix), such as `my-${index}-name`. When OpenSearchSink is sending data to OpenSearch, the format portion "${index}" will be replaced by it's value in the event that is being processed. The format may also be like "${index1/index2/index3}" in which case the field "index1/index2/index3" is searched in the event and replaced by its value.
     - Additionally, the formatted string can include expressions to evaluate to format the index name. For example, `my-${index}-${getMetadata(\"some_metadata_key\")}-name` will inject both the `index` value from the Event, as well as the value of `some_metadata_key` from the Event metadata to construct the index name.
-- <a name="template_type"></a>`template_type`(optional): Defines what type of OpenSearch template to use. The available options are `v1` and `index-template`. The default value is `v1`, which uses the original OpenSearch templates available at the `_template` API endpoints. Select `index-template` to use composable index templates which are available at OpenSearch's `_index_template` endpoint.
+- <a name="template_type"></a>`template_type`(optional): Defines what type of OpenSearch template to use. The available options are `v1` and `index-template`. The default value is `v1`, which uses the original OpenSearch templates available at the `_template` API endpoints. Select `index-template` to use composable index templates which are available at OpenSearch's `_index_template` endpoint. Note: when `distribution_version` is `es6`, `template_type` is enforced into `v1`.
 
 - <a name="template_file"></a>`template_file`(optional): A json file path or AWS S3 URI to be read as index template for custom data ingestion. The json file content should be the json value of
 `"template"` key in the json content of OpenSearch [Index templates API](https://opensearch.org/docs/latest/opensearch/index-templates/),