chore(ecs): remove deprecated method from readme (#20246)

This PR replace `cluster.addAutoScalingGroup()` to `cluster.addAsgCapacityProvider()` because `addAutoScalingGroup()` is deprecated.

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: yamatatsu

packages/@aws-cdk/aws-ecs/README.md

@@ -139,7 +139,10 @@ const autoScalingGroup = new autoscaling.AutoScalingGroup(this, 'ASG', {
   // ... other options here ...
 });
 
-cluster.addAutoScalingGroup(autoScalingGroup);
+const capacityProvider = new ecs.AsgCapacityProvider(this, 'AsgCapacityProvider', {
+  autoScalingGroup,
+});
+cluster.addAsgCapacityProvider(capacityProvider);
 ```
 
 If you omit the property `vpc`, the construct will create a new VPC with two AZs.