Update adding-instance-properties.md (vuejs#2372)

Fix Broken Link

Change https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/this-object-prototypes/ch5.md to https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/objects-classes/ch5.md

Author: ikrydev

src/v2/cookbook/adding-instance-properties.md

@@ -58,7 +58,7 @@ new Vue({
 })
 ```
 
-It would be `"My App"`, then `"The name of some other app"`, because `this.appName` is overwritten ([sort of](https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/this-object-prototypes/ch5.md)) by `data` when the instance is created. We scope instance properties with `$` to avoid this. You can even use your own convention if you'd like, such as `$_appName` or `ΩappName`, to prevent even conflicts with plugins or future features.
+It would be `"My App"`, then `"The name of some other app"`, because `this.appName` is overwritten ([sort of](https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/objects-classes/ch5.md)) by `data` when the instance is created. We scope instance properties with `$` to avoid this. You can even use your own convention if you'd like, such as `$_appName` or `ΩappName`, to prevent even conflicts with plugins or future features.
 
 ## Real-World Example: Replacing Vue Resource with Axios