Update dns_zone.html.markdown (hashicorp#8273)

Author: ryanmoran

website/source/docs/providers/azurerm/r/dns_zone.html.markdown

@@ -39,6 +39,7 @@ The following attributes are exported:
 * `id` - The DNS Zone ID.
 * `max_number_of_record_sets` - (Optional) Maximum number of Records in the zone. Defaults to `1000`.
 * `number_of_record_sets` - (Optional) The number of records already in the zone.
+* `name_servers` - (Optional) A list of values that make up the NS record for the zone.
 
 
 ## Import
@@ -47,4 +48,4 @@ DNS Zones can be imported using the `resource id`, e.g.
 
 ```
 terraform import azurerm_dns_zone.zone1 /subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/mygroup1/providers/Microsoft.Network/dnsZones/zone1
-```
+```