lua: typo

`LoudDog.__index` is `Dog`, not `LoudDog`.

----

Test code:

```
; lua
> Dog = {}
> function Dog:new()
>> local newObj = {sound = 'woof'}
>> self.__index = self
>> return setmetatable(newObj, self)
>> end
> LoudDog = Dog:new()
> if LoudDog.__index == Dog and LoudDog.__index
 ~= LoudDog  then print 'true' end
true
```