fix the document on enveloped object

Can you fix the document at docs/howtos/envelope-object.md to reflect that we have changed from using a configMap to use CRDs defined in apis/placement/v1beta1/envelope_types.go, you can use the example yamls under the directory examples/envelopes to illustrate the usage. Please be aware that there are two types of envelope objects now, one is called "ClusterResourceEnvelope" that can only wrap cluster scoped resources and the other is "ResourceEnvelope" that can only wrap namespace scoped resources. Thank you very much.