Replace tag with in app3 (vuejs#1149) (vuejs#356)

* tweak ad styling

* Replace <p> tag with <span> in app3 (vuejs#1149)

The code snippet uses `<p>`, but the actual demo uses `<span>`

Author: vuejs-jp-bot

src/v2/guide/index.md

@@ -100,7 +100,7 @@ var app2 = new Vue({
 
 ``` html
 <div id="app-3">
-  <p v-if="seen">Now you see me</p>
+  <span v-if="seen">Now you see me</span>
 </div>
 ```