change host to host_migration

host param in migration refer to wrong link.

Part of bp:api-ref-in-rst

Change-Id: I7bf2459dace2cf0a90ae32f75ed734f533219c63

api-ref/source/parameters.yaml

@@ -1024,7 +1024,7 @@ hostId:
   in: body
   required: true
   type: string
-host_2:
+host_migration:
   description: |
     The host to which to migrate the server. If you omit this parameter, the scheduler
     chooses a host.

api-ref/source/servers-admin-action.inc

@@ -137,7 +137,7 @@ Request
 
   - tenant_id: tenant_id
   - server_id: server_id
-  - host: host
+  - host: host_migration 
   - block_migration: block_migration
   - disk_over_commit: disk_over_commit