just added credits to a tutorial / guide (#4962)

free-programming-books.md

@@ -282,7 +282,7 @@
 * [DirectX manual](http://user.xmission.com/~legalize/book/download/index.html) (draft)
 * [GPU Gems](https://developer.nvidia.com/gpugems/GPUGems/gpugems_pref01.html)
 * [Graphics Programming Black Book](http://www.gamedev.net/page/resources/_/technical/graphics-programming-and-theory/graphics-programming-black-book-r1698) - Michael Abrash
-* [Introduction to Modern OpenGL](https://open.gl)
+* [Introduction to Modern OpenGL](https://open.gl) - Alexander Overvoorde (HTML, EPUB, PDF) (C++)
 * [Introduction to TouchDesigner 099](https://leanpub.com/introductiontotouchdesigner/) *(Leanpub account or valid email requested)*
 * [Learn Computer Graphics From Scratch!](https://www.scratchapixel.com) - Scratchapixel (:construction: *in process*)
 * [Learn OpenGL](http://learnopengl.com) - Joey de Vries