WorkGroup tags schema is incorrect

[iliapolo]

According to the spec, the tags for a workgroup should be specified like so:

"Tags": {
  "Tags": [{"Key": "foo", "Value": "bar"}]
}

However, this seems to result in an Internal Error upon deployment.

Instead, it should be:

"Tags": [{"Key": "foo", "Value": "bar"}]

Which does make more sense.

Originally discovered in aws/aws-cdk#6936