Fix "Creates an aggregate" parameters

"Creates an aggregate" API supports multiple parameters on a request,
but api-ref doesn.t contain them at all This patch adds them.

The following JSON-Schema is for the API.
https://github.com/openstack/nova/blob/master/nova/api/openstack/compute/schemas/aggregates.py#L32

Part of bp:api-ref-in-rst

Change-Id: I3a58568662d21059bccdcf64cc30dbf3b706db58

api-ref/source/os-aggregates.inc

@@ -26,6 +26,8 @@ Request
 .. rest_parameters:: parameters.yaml
 
   - tenant_id: tenant_id
+  - name: aggregate_name
+  - availability_zone: availability_zone
 
 **Example Create Aggregate: JSON request**