org-mime.el: Fix org-mime-htmlize

* org-mime.el (org-mime-htmlize): Make argument optional.

contrib/lisp/org-mime.el

@@ -194,10 +194,10 @@ and images in a multipart/related part."
       str)
      html-images)))
 
-(defun org-mime-htmlize (arg)
+(defun org-mime-htmlize (&optional arg)
   "Export a portion of an email body composed using `mml-mode' to
-html using `org-mode'.  If called with an active region only
-export that region, otherwise export the entire body."
+  html using `org-mode'.  If called with an active region only
+  export that region, otherwise export the entire body."
   (interactive "P")
   (require 'ox-org)
   (require 'ox-html)