Update free-programming-interactive-tutorials-en.md (#4679)

* Update free-programming-interactive-tutorials-en.md

* use resource title, remove trailing slash

Co-authored-by: Eric Hellman <eric@hellman.net>

free-programming-interactive-tutorials-en.md

@@ -167,6 +167,7 @@
 * [Javascripting](https://github.com/sethvincent/javascripting)
 * [Learn JavaScript](http://www.learn-js.org)
 * [Learn knockout.js](http://learn.knockoutjs.com)
+* [Learn to Code for Free – Grasshopper](https://grasshopper.app)
 * [Learning Advanced JavaScript](http://ejohn.org/apps/learn/)
 * [Try jQuery](http://try.jquery.com)