Update README.md

Added `name` field to second example. There is a known bug cloudposse#151 if the name field is ommited. Might be easier to just update the docs than fix the bug, since it's been open since 2021.

README.md

@@ -86,6 +86,7 @@ module "cdn" {
   aliases           = ["assets.cloudposse.com"]
   dns_alias_enabled = true
   parent_zone_name  = "cloudposse.com"
+  name              = "prod"
 }
 ```