Playground background wrong color in documentation

[n8behavior]

This is quite pedantic but it did make me think I had my setup wrong.

In the section on Building the Tutorials
http://www.opengl-tutorial.org/beginners-tutorials/tutorial-1-opening-a-window/#building-the-tutorials

For each environment the docs say you should see a black background but the code actually sets it to blue
https://github.com/opengl-tutorials/ogl/blob/master/playground/playground.cpp#L51

Again, super picky but might confuse others as it did me. OTOH, it got me into the code right away! So... :D

Also, I'd be happy to send a PR but not sure where the tutorial documents are...only see the code in here.

BTW, these are fantastic...thank you!!