Update index.md (vuejs#1036)

Author: Alex-Sokolov

src/v2/api/index.md

@@ -705,7 +705,7 @@ if (version === 2) {
 
 ## Options / Lifecycle Hooks
 
-All lifecycle hooks automatically have their `this` context bound to the instance, so that you can access data, computed properties, and methods. This means __you should not use an arrow function to define a lifecycle method__ (e.g. `created: () => this.fetchTodos()`). The reason is arrow functions bind the parent context, so `this` will not be the Vue instance as you expect and `this.fetchTodos` will be undefined.
+<p class="tip">All lifecycle hooks automatically have their `this` context bound to the instance, so that you can access data, computed properties, and methods. This means __you should not use an arrow function to define a lifecycle method__ (e.g. `created: () => this.fetchTodos()`). The reason is arrow functions bind the parent context, so `this` will not be the Vue instance as you expect and `this.fetchTodos` will be undefined.</p>
 
 ### beforeCreate