Fixed: the URL link to the style guide

Was: https://github.com/mobify/mobify-code-style/blob/master/css-handbook/Readme.md
Now: https://github.com/mobify/mobify-code-style/blob/master/css/Readme.md

@jamesbull
Merge pull request #11 from allanchang/patch-1

Fixed: the URL link to the style guide