Update the elisp blurb in the README

[BenQuigley]

Thanks for this repo. We could make a couple updates to the example blurb:

;; .doom.d/config.el
(let ((alternatives '("nameoffirstfile.svg"
                      "nameofanotherfile.png"
                      "nameofathirdfile.svg")))
  (setq fancy-splash-image
        (concat doom-private-dir "splash/"
                (nth (random (length alternatives)) alternatives))))

I wanted to be able to just drop my files in the splash dir, so I took this and updated it to:

;; Pick a random splash image from {user-dir}/splash-images/.
(let* ((splash-dir (concat doom-user-dir "splash-images/"))
       (files (directory-files splash-dir t "\\(\\.png\\|\\.svg\\|\\.jpe?g\\)$")))
  (setq fancy-splash-image
        (nth (random (length files)) files)))

Let me know if you'd like a PR, or feel free to add this version yourself if you like it.