Fix closing src tag in TUTORIAL.org (#767)

Should use `#+end_src` instead of `#+end` for closing tag.

Author: gongzhitaao

TUTORIAL.org

@@ -13,7 +13,7 @@ Example:
 
   ;; or bind it to the function directly
   (setq meow--kbd-forward-char #'forward-char)
-#+END
+#+END_SRC
 
 * Keybindings Cheat Sheet