Update article.md

Add "the" before "'this' keyword".

1-js/04-object-basics/04-object-methods/article.md

@@ -98,7 +98,7 @@ It's common that an object method needs to access the information stored in the
 
 For instance, the code inside `user.sayHi()` may need the name of the `user`.
 
-**To access the object, a method can use `this` keyword.**
+**To access the object, a method can use the `this` keyword.**
 
 The value of `this` is the object "before dot", the one used to call the method.