SDL corrections in content creation chapter

SDL doesn't require full `main()` prototype since SDL 2. see http://wiki.libsdl.org/MigrationGuide > *Some general thruths*

Then I couldn't help but mention that you don't have to specify position for window, you can let OS' WM choose or place it in the center.

Also you were missing two `are`s in the same paragraph.

What's left is that afaik you don't need to link against SDL2main, because I don't and everything works. I can't check on windows though.