ox-koma-letter: Fix docstring

* ox-koma-letter.el (org-koma-letter-special-tags-as-macro): Fix docstring.

contrib/lisp/ox-koma-letter.el

@@ -383,7 +383,7 @@ e.g. \"title-subject:t\"."
   "Header tags to be inserted after closing.")
 
 (defconst org-koma-letter-special-tags-after-letter '(after_letter)
-  "Header tags to be inserted after closing.")
+  "Header tags to be inserted after the letter.")
 
 (defvar org-koma-letter-special-contents nil
   "Holds special content temporarily.")