fix: update the alt default value with correct type (#446)

Author: q2w

modules/instance_template/metadata.display.yaml

@@ -34,8 +34,8 @@ spec:
           altDefaults:
             - type: ALTERNATE_TYPE_DC
               value:
-                nat_ip: null
-                network_tier: PREMIUM
+                - nat_ip: null
+                  network_tier: PREMIUM
         additional_disks:
           name: additional_disks
           title: Additional Disks

modules/mig/metadata.yaml

@@ -296,6 +296,9 @@ spec:
         type: string
       - name: instance_group_manager
         description: An instance of google_compute_region_instance_group_manager of the instance group.
+        type:
+          - object
+          - base_instance_name: string
       - name: self_link
         description: Self-link of managed instance group
         type: string