org-keys: Use RET instead of <return> where possible

* lisp/org-keys.el (org-mode-map): Use RET instead of <return> where
  possible.

Reported-by: Kévin Le Gouguec <kevin.legouguec@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2019-01/msg00162.html>

lisp/org-keys.el

@@ -449,9 +449,9 @@ COMMANDS is a list of alternating OLDDEF NEWDEF command names."
 (define-key org-mode-map (kbd "<backtab>") #'org-shifttab)
 
 ;;;; RET key with modifiers
-(org-defkey org-mode-map (kbd "S-<return>") #'org-table-copy-down)
-(org-defkey org-mode-map (kbd "M-S-<return>") #'org-insert-todo-heading)
-(org-defkey org-mode-map (kbd "ESC S-<return>") #'org-insert-todo-heading)
+(org-defkey org-mode-map (kbd "S-RET>") #'org-table-copy-down)
+(org-defkey org-mode-map (kbd "M-S-RET") #'org-insert-todo-heading)
+(org-defkey org-mode-map (kbd "ESC S-RET") #'org-insert-todo-heading)
 (org-defkey org-mode-map (kbd "M-RET") #'org-meta-return)
 (org-defkey org-mode-map (kbd "ESC RET") #'org-meta-return)