api-ref: Adding backup create response example

The backup create response json file exists but isn't linked
with the api-ref guide.
This patch adds the link to it.

Change-Id: Ib26670803ed57a23f282ac8f679db6e4957e8cc7

api-ref/source/v3/ext-backups.inc

@@ -311,6 +311,11 @@ Response Parameters
    - links: links_1
    - name: name_1
 
+Response Example
+----------------
+
+.. literalinclude:: ./samples/backup-create-response.json
+   :language: javascript
 
 Update a backup
 ~~~~~~~~~~~~~~~