ox-koma-letter: Fix a docstring

* contrib/lisp/ox-koma-letter.el (org-koma-letter-closing): Fix
  docstring.

contrib/lisp/ox-koma-letter.el

@@ -181,7 +181,7 @@ t the value opening will be implicit set as the headline title."
   :type 'string)
 
 (defcustom org-koma-letter-closing ""
-  "Koma-Letter's closing, as a string."
+  "Letter's closing, as a string."
   :group 'org-export-koma-letter
   :type 'string)