Fix broken link

[soloko]

https://github.com/EightShapes/contrast-grid/blob/master/dist/index.html#L141

this href="http://www.eightshapes.com/design-systems.html" should be href="http://www.eightshapes.com/design-systems"

I'm guessing that .html was forgotten

[rianmurnen]

If this change is address, also consider updating to the secure protocol (i.e. https).