missing argument to error message

* contrib/lisp/ox-koma-letter.el (org-koma-letter-template): Missing
  argument to error message.

contrib/lisp/ox-koma-letter.el

@@ -510,7 +510,7 @@ holding export options."
 		      "^[ \t]*\\\\documentclass\\(\\(\\[[^]]*\\]\\)?\\)"
 		      class-options header t nil 1)))))
         (if (not document-class-string)
-	    (user-error "Unknown LaTeX class `%s'")
+	    (user-error "Unknown LaTeX class `%s'" class)
           (org-latex-guess-babel-language
            (org-latex-guess-inputenc
             (org-splice-latex-header