api-ref: Fix a parameter in os-availability-zone.inc

In "Get Availability Zone Information",
the 'hosts' response parameter is always 'null'.
So fix the description.

Change-Id: I23bd8b3a422aa03c3f56d7f2f10f6603acd0078a
Closes-Bug: #1654427

api-ref/source/os-availability-zone.inc

@@ -30,7 +30,7 @@ Response
 .. rest_parameters:: parameters.yaml
 
   - availabilityZoneInfo: availability_zone_info
-  - hosts: hosts.availability_zone
+  - hosts: hosts.availability_zone_none
   - zoneName: OS-EXT-AZ:availability_zone
   - zoneState: availability_zone_state
   - available: available

api-ref/source/parameters.yaml

@@ -2263,6 +2263,12 @@ hosts.availability_zone:
   in: body
   required: true
   type: object
+hosts.availability_zone_none:
+  description: |
+    It is always ``null``.
+  in: body
+  required: true
+  type: none
 hours:
   description: |
    The duration that the server exists (in hours).