fix dotfiles link

Closes https://github.com/afeld/git-plugins/pull/15.

@tarebyte I'd rather have the permalink since the contents of that file could change and then those line numbers wouldn't be right, but thanks for pointing it out!  Not sure why that old commit isn't present...

![shifty eyes](http://26.media.tumblr.com/tumblr_lkfhpg4LZu1qh59n0o1_500.gif)