org: Fix typo

* lisp/org.el (org-not-done-heading-regexp): Fix a typo and make
  variable buffer-local, as expected.

Reported-by: Alexander Borkowski <alex-b@opus-b.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/97996>

lisp/org.el

@@ -4802,7 +4802,7 @@ Otherwise, these types are allowed:
 (make-variable-buffer-local 'org-not-done-regexp)
 (defvar org-not-done-heading-regexp nil
   "Matches a TODO headline that is not done.")
-(make-variable-buffer-local 'org-not-done-regexp)
+(make-variable-buffer-local 'org-not-done-heading-regexp)
 (defvar org-todo-line-regexp nil
   "Matches a headline and puts TODO state into group 2 if present.")
 (make-variable-buffer-local 'org-todo-line-regexp)