Add missing custom var type

Author: purcell

purescript-font-lock.el

@@ -114,7 +114,8 @@
     ("::" . ,(decode-char 'ucs 8759))
     ("forall" . ,(decode-char 'ucs 8704)))
   "A set of symbol compositions for use as `prettify-symbols-alist'."
-  :group 'purescript)
+  :group 'purescript
+  :type '(repeat (cons string character)))
 
 ;; Use new vars for the font-lock faces.  The indirection allows people to
 ;; use different faces than in other modes, as before.