fix Backbone.sync mock logging example

Current example always only created items, because an "id" was not set
on the item. With this addition, items can go through the cycle of
create, update, delete.

index.md

@@ -942,8 +942,10 @@ var methodMap = {
 In the above example if we wanted to log an event when `.sync()` was called, we could do this:
 
 ```javascript
+var id_counter = 1;
 Backbone.sync = function(method, model) {
   console.log("I've been passed " + method + " with " + JSON.stringify(model));
+  if(method === 'create'){ model.set('id', id_counter++); }
 };
 ```