Use better URL for OpenGL tutorial (fixes #34 again)

Overv · Overv · commit 948ff2677320 · 2018-10-21T14:57:06.000+02:00
diff --git a/00_Introduction.md b/00_Introduction.md
@@ -38,7 +38,7 @@ tutorial:
 
 This tutorial will not assume knowledge of OpenGL or Direct3D concepts, but it
 does require you to know the basics of 3D computer graphics. It will not explain
-the math behind perspective projection, for example. See [this online book](http://opengl.datenwolf.net/gltut/html/index.html)
+the math behind perspective projection, for example. See [this online book](https://paroj.github.io/gltut/)
 for a great introduction of computer graphics concepts.
 
 You can use C instead of C++ if you want, but you will have to use a different
