Improve document usability (#816)

Replace long lines in sample code with short lines to avoid horizontal scrollbars

Author: zrw354

src/v2/guide/events.md

@@ -144,7 +144,9 @@ new Vue({
 Sometimes we also need to access the original DOM event in an inline statement handler. You can pass it into a method using the special `$event` variable:
 
 ``` html
-<button v-on:click="warn('Form cannot be submitted yet.', $event)">Submit</button>
+<button v-on:click="warn('Form cannot be submitted yet.', $event)">
+  Submit
+</button>
 ```
 
 ``` js