Fix a minor error in API reference

Based on test result, response body of claim messages API
includes a top level key "messages" whose vlaue is a list of
messages being successfully claimed. This patch revises
corresponding sample in API reference.

Change-Id: I72b7b7a59e9c54c398448e474d2d13f77454e29c

api-ref/source/samples/claim_messages_response.json

@@ -1,4 +1,5 @@
-[
+{
+    "messages": [
         {
             "body": {
                 "event": "BackupStarted"
@@ -8,3 +9,4 @@
             "ttl": 300
         }
     ]
+}