Make prefix argument optional

It seems to be the general style of how prefix args are done...

core/prelude-core.el

@@ -550,7 +550,7 @@ This follows freedesktop standards, should work in X servers."
                              '(2 "_NET_WM_STATE_FULLSCREEN" 0))
     (error "Only X server is supported")))
 
-(defun prelude-find-user-init-file (arg)
+(defun prelude-find-user-init-file (&optional arg)
   "Edit the `prelude-user-init-file', in another window.
 With a prefix argument ARG, find the `user-init-file' instead."
   (interactive "P")