Update free-programming-books.md

changed "Graphics User Interfaces" into "Graphical User Interfaces"

free-programming-books.md

 ###Index
 * [Meta-Lists](#meta-lists)
-* [Graphics user interfaces](#graphics-user-interfaces)
+* [Graphical user interfaces](#graphical-user-interfaces)
 * [Graphics Programming](#graphics-programming)
 * [Language Agnostic](#language-agnostic)
     * [Algorithms & Datastructures](#algorithms--datastructures)
@@ -160,7 +160,7 @@
 * [Tutorials for modern OpenGL](http://www.opengl-tutorial.org/)
 * [OpenGL Programming Guide (The Red Book)](http://fly.srk.fer.hr/~unreal/theredbook/)
 
-###Graphics User Interfaces
+###Graphical User Interfaces
 * [Programming with gtkmm 3](https://developer.gnome.org/gtkmm-tutorial/stable/)