this's value in setup is undefined (#1644)

Author: Huauauaa

src/api/composition-api-setup.md

@@ -41,7 +41,7 @@ export default {
 Note that [refs](/api/reactivity-core.html#ref) returned from `setup` are [automatically shallow unwrapped](/guide/essentials/reactivity-fundamentals.html#deep-reactivity) when accessed in the template so you do not need to use `.value` when accessing them. They are also unwrapped in the same way when accessed on `this`.
 
 :::tip
-`setup()` itself does not have access to the component instance - `this` will have a value of `null` inside `setup()`. You can access Composition-API-exposed values from Options API, but not the other way around.
+`setup()` itself does not have access to the component instance - `this` will have a value of `undefined` inside `setup()`. You can access Composition-API-exposed values from Options API, but not the other way around.
 :::
 
 ## Accessing Props