ox-s5: Don't use org-html-end-plain-list

* contrib/lisp/ox-s5.el (org-s5-plain-list): Adjust for the removal of
org-html-end-plain-list.

The function org-html-end-plain-list was deleted in
8855c23c6 (ox-html: Plain list supports arbitrary attributes,
2017-02-13).

Reported-by: <dchechin92@gmail.com>
<https://lists.gnu.org/archive/html/emacs-orgmode/2017-10/msg00123.html>

contrib/lisp/ox-s5.el

@@ -292,7 +292,8 @@ which will make the list into a \"build\"."
              "<%s class='org-%s%s'>" tag tag
              (if (org-export-get-node-property :INCREMENTAL plain-list t)
                  " incremental" ""))
-            contents (org-html-end-plain-list type))))
+            contents
+	    (format "</%s>" tag))))
 
 (defun org-s5-inner-template (contents info)
   "Return body of document string after HTML conversion.