* lisp/org/ob-lisp.el (org-babel-lisp-eval-fn): Tweak type.

Avoids type mismatch when relevant library is not present/loaded.

lisp/org/ob-lisp.el

@@ -54,7 +54,7 @@ Valid values include `slime-eval' and `sly-eval'."
   :group 'org-babel
   :version "26.1"
   :package-version '(Org . "9.0")
-  :type 'function)
+  :type 'symbol)
 
 (defcustom org-babel-lisp-dir-fmt
   "(let ((*default-pathname-defaults* #P%S\n)) %%s\n)"