org-mime.el: Set `org-export-with-LaTeX-fragments' correctly.

* org-mime.el (org-mime-htmlize): Set
`org-export-with-LaTeX-fragments' correctly.

contrib/lisp/org-mime.el

@@ -206,7 +206,7 @@ export that region, otherwise export the entire body."
          ;; makes the replies with ">"s look nicer
          (org-export-preserve-breaks org-mime-preserve-breaks)
 	 ;; dvipng for inline latex because MathJax doesn't work in mail
-	 (org-export-with-LaTeX-fragments "dvipng")
+	 (org-export-with-LaTeX-fragments 'dvipng)
          ;; to hold attachments for inline html images
          (html-and-images
           (org-mime-replace-images