update one code comment in 1-js/09-classes/04-private-protected-properties-methods

It looks like code was changed some time ago, but comments didn't change accordingly.

Author: ZYinMD

1-js/09-classes/04-private-protected-properties-methods/article.md

@@ -116,7 +116,7 @@ class CoffeeMachine {
 let coffeeMachine = new CoffeeMachine(100);
 
 // add water
-coffeeMachine.waterAmount = -10; // Error: Negative water
+coffeeMachine.waterAmount = -10; // _waterAmount will become 0, not -10
 ```
 
 Now the access is under control, so setting the water amount below zero becomes impossible.