Update article.md

Adjustments to line about class properties. Add "the", change "the" to "a".

Author: paroche

1-js/09-classes/01-class/article.md

@@ -320,7 +320,7 @@ class User {
 new User().sayHi();
 ```
 
-The property `name` is not placed into `User.prototype`. Instead, it is created by `new` before calling constructor, it's the property of the object itself.
+The property `name` is not placed into `User.prototype`. Instead, it is created by `new` before calling the constructor, it's a property of the object itself.
 
 ## Summary