Update ECS configuration docs (#3858)

docs/content/en/docs-dev/user-guide/configuration-reference.md

@@ -429,7 +429,7 @@ One of `yamlField` or `regex` is required.
 |-|-|-|-|
 | serviceDefinitionFile | string | The path ECS Service configuration file. Allow file in both `yaml` and `json` format. The default value is `service.json`. See [here](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service_definition_parameters.html) for parameters.| No |
 | taskDefinitionFile | string | The path to ECS TaskDefinition configuration file. Allow file in both `yaml` and `json` format. The default value is `taskdef.json`. See [here](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html) for parameters. | No |
-| targetGroups | [ECSTargetGroupInput](#ecstargetgroupinput) | The target groups configuration, will be used to routing traffic to created task sets. | Yes |
+| targetGroups | [ECSTargetGroupInput](#ecstargetgroupinput) | The target groups configuration, will be used to routing traffic to created task sets. | Yes (if you want to perform progressive delivery) |
 
 ### ECSTargetGroupInput