fix a typo

add "is"

Author: shintaroid

1-js/05-data-types/04-array/article.md

@@ -4,7 +4,7 @@ Objects allow to store keyed collections of values. That's fine.
 
 But quite often we find that we need an *ordered collection*, where we have a 1st, a 2nd, a 3rd element and so on. For example, we need that to store a list of something: users, goods, HTML elements etc. 
 
-It not convenient to use an object here, because it provides no methods to manage the order of elements. We can’t insert a new property “between” the existing ones. Objects are just not meant for such use.
+It is not convenient to use an object here, because it provides no methods to manage the order of elements. We can’t insert a new property “between” the existing ones. Objects are just not meant for such use.
 
 There exists a special data structure named `Array`, to store ordered collections.