v3(docs): Fix service credential binding list example

[#174252663](https://www.pivotaltracker.com/story/show/174252663)

Author: pivotal-marcela-campo

docs/v3/source/includes/api_resources/_service_credential_bindings.erb

@@ -152,7 +152,12 @@
       "updated_at": "2016-06-08T16:41:26Z",
       "name": "some-key-name",
       "type": "key",
-      "last_operation": null,
+      "last_operation": {
+        "type": "create",
+        "state": "succeeded",
+        "created_at": "2015-11-13T17:02:56Z",
+        "updated_at": "2016-06-08T16:41:26Z"
+      },
       "relationships": {
         "service_instance": {
           "data": {