Fix wrong statement about arrow function and bound this (#2040)

* Fix wrong statement about arrow function and bound this

Stating that arrow functions are bound to it's parent `this` is wrong, arrow functions does not have a `this` at all. More: https://twitter.com/getify/status/1101521219243966466

* Update src/v2/guide/instance.md

Co-Authored-By: KadoBOT <ricardo.ambrogi@gmail.com>

* Update instance.md

Remove unnecessary `a`

Author: KadoBOT

src/v2/guide/instance.md

@@ -142,7 +142,7 @@ new Vue({
 
 There are also other hooks which will be called at different stages of the instance's lifecycle, such as [`mounted`](../api/#mounted), [`updated`](../api/#updated), and [`destroyed`](../api/#destroyed). All lifecycle hooks are called with their `this` context pointing to the Vue instance invoking it.
 
-<p class="tip">Don't use [arrow functions](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions) on an options property or callback, such as `created: () => console.log(this.a)` or `vm.$watch('a', newValue => this.myMethod())`. Since arrow functions are bound to the parent context, `this` will not be the Vue instance as you'd expect, often resulting in errors such as `Uncaught TypeError: Cannot read property of undefined` or `Uncaught TypeError: this.myMethod is not a function`.</p>
+<p class="tip">Don't use [arrow functions](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions) on an options property or callback, such as `created: () => console.log(this.a)` or `vm.$watch('a', newValue => this.myMethod())`. Since an arrow function doesn't have a `this`, `this` will be treated as any other variable and lexically looked up through parent scopes until found, often resulting in errors such as `Uncaught TypeError: Cannot read property of undefined` or `Uncaught TypeError: this.myMethod is not a function`.</p>
 
 ## Lifecycle Diagram