Fix the melpa link on the README

[tshrinivasan]

(add-to-list 'package-archives '("melpa" . "http://melpa.milkbox.net/packages/") t)

This should be

(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)

[ahungry]

Thanks @tshrinivasan - that's got to be an ancient reference 😆