Proper capitalization in config.js

[Perseus333]

I was creating my own fork and I wanted to try out different styles, however, when I wanted to switch back to OneDark I noticed that the icon wasn't loading properly. This was because I was trying to access Onedark as the comment in config.js suggests instead of the actual path OneDark with capital D. I think that this is an easy fix, but has caused me a bit of headaches.