tests/resource/aws_elasticsearch_domain: Fix configuration issues in TestAccAWSElasticSearchDomain_vpc_update (#15180)
tests/resource/aws_elasticsearch_domain: Fix configuration issues in TestAccAWSElasticSearchDomain_vpc_update (#15180)

Previously:

```
=== CONT  TestAccAWSElasticSearchDomain_vpc_update
TestAccAWSElasticSearchDomain_vpc_update: resource_aws_elasticsearch_domain_test.go:415: Step 1/3 error: terraform failed: exit status 1
stderr:
Error: Missing item separator
on config717422516/terraform_plugin_test.tf line 83, in resource "aws_elasticsearch_domain" "test":
83:     subnet_ids         = [aws_subnet.az1_ first.id, aws_subnet.az2_ first.id]
Expected a comma to mark the beginning of the next item.
--- FAIL: TestAccAWSElasticSearchDomain_vpc_update (1.46s)
```

Output from acceptance testing:

```
--- PASS: TestAccAWSElasticSearchDomain_vpc_update (1889.93s)
```
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws