Update index.md (vuejs#808)

Break long lines in sample HTML code into short lines to avoid a horizontal scrollbar caused by limited column width

Author: zrw354

src/v2/guide/index.md

@@ -60,7 +60,8 @@ In addition to text interpolation, we can also bind element attributes like this
 ``` html
 <div id="app-2">
   <span v-bind:title="message">
-    Hover your mouse over me for a few seconds to see my dynamically bound title!
+    Hover your mouse over me for a few seconds
+    to see my dynamically bound title!
   </span>
 </div>
 ```