doc: add missing mac_address attribute in the instance_private_nic (#771)

Author: Mayeu

docs/resources/instance_private_nic.md

@@ -30,6 +30,7 @@ The following arguments are required:
 In addition to all above arguments, the following attributes are exported:
 
 - `id` - The ID of the private NIC.
+- ``mac_address` - The MAC address of the private NIC.
 
 ## Import