Docstring fix for org-export-preprocess-string

Hi,

I found a trivial error with this docstring.

--8<---------------cut here---------------start------------->8---
--8<---------------cut here---------------end--------------->8---

Best regards,
  Seb

lisp/org-exp.el

@@ -1042,7 +1042,7 @@ Pressing `1' will switch between these two options."
   "Alist of code references and line numbers.")
 
 (defun org-export-preprocess-string (string &rest parameters)
-  "Cleanup STRING so that that the true exported has a more consistent source.
+  "Cleanup STRING so that the true exported has a more consistent source.
 This function takes STRING, which should be a buffer-string of an org-file
 to export.  It then creates a temporary buffer where it does its job.
 The result is then again returned as a string, and the exporter works
@@ -3084,4 +3084,3 @@ The depends on the variable `org-export-copy-to-kill'."
 ;; arch-tag: 65985fe9-095c-49c7-a7b6-cb4ee15c0a95
 
 ;;; org-exp.el ends here
-