Correct app reference in 2.0 docs (#395)

Author: okcscott

src/guide/index.md

@@ -113,7 +113,7 @@ var app3 = new Vue({
 </script>
 {% endraw %}
 
-Go ahead and enter `app2.seen = false` in the console. You should see the message disappear.
+Go ahead and enter `app3.seen = false` in the console. You should see the message disappear.
 
 This second example demonstrates that we can bind data to not only text and attributes, but also the **structure** of the DOM. Moreover, Vue also provides a powerful transition effect system that can automatically apply [transition effects](transitions.html) when elements are inserted/updated/removed by Vue.