Update free-programming-books-ko.md (#5349)

* Update free-programming-books-ko.md

* Remove trailling slash

books/free-programming-books-ko.md

@@ -86,6 +86,7 @@
 ### JavaScript
 
 * [JavaScript Garden](http://bonsaiden.github.io/JavaScript-Garden/ko)
+* [JavaScript로 만나는 세상](https://helloworldjavascript.net)
 
 
 #### Node.js