docs(ecs): describe default tag value for fromEcrRepository method (#30926)

### Description of changes

If we don't specify the tag parameter for fromEcrRepository method, `latest` is used as default value.
https://github.com/aws/aws-cdk/blob/c8e5924bbc83b91b929838518f4955dd3bbb884f/packages/aws-cdk-lib/aws-ecs/lib/container-image.ts#L19-L24

However, currently this behavior is not described in document.
https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ecs.ContainerImage.html#static-fromwbrecrwbrrepositoryrepository-tag

I'm adding the description regarding this default value.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: tam0ri

Diff for: packages/aws-cdk-lib/aws-ecs/lib/container-image.ts

@@ -18,6 +18,8 @@ export abstract class ContainerImage {
 
   /**
    * Reference an image in an ECR repository
+   *
+   * @param tag If you don't specify this parameter, `latest` is used as default.
    */
   public static fromEcrRepository(repository: ecr.IRepository, tag: string = 'latest') {
     return new EcrImage(repository, tag);