New PDF book added - Developing 2D Games with Unity (#9270)

* New book added - Developing 2D Games with Unity]

* Update with new ebook - Level up your code with game programming patterns

* Space Removed

books/free-programming-books-subjects.md

@@ -293,6 +293,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Game AI Pro](http://www.gameaipro.com) - Steve Rabin
 * [Game Design with AGS](https://ensadi.github.io/AGSBook/) - Dave Ensminger, A. G. Madi
 * [Game Programming Patterns](http://gameprogrammingpatterns.com) - Bob Nystrom
+* [Level up your code with game programming patterns](https://resources.unity.com/games/level-up-your-code-with-game-programming-patterns) - Unity (HTML & PDF)
 * [Procedural Content Generation in Games](http://pcgbook.com) - Noor Shaker, Julian Togelius, Mark Nelson