Update article.md

Author: david-cho-lerat-HL2

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

@@ -86,7 +86,7 @@ alert(typeof User); // function
 
 What `class User {...}` construct really does is:
 1. Creates a function named `User`, that becomes the result of the class declaration.
-    - The function code is taken from the `constructor` method (assumed empty is we don't write such method).
+    - The function code is taken from the `constructor` method (assumed empty if we don't write such method).
 3. Stores all methods, such as `sayHi`, in `User.prototype`.
 
 Afterwards, for new objects, when we call a method, it's taken from the prototype, just as  described in the chapter <info:function-prototype>. So `new User` object has access to class methods.