Fix implementation of `para`

Commit e347108444895bd86c5ee07f2ab0a424593dc821 broke `para`:

```
import Data.Functor.Foldable
import Data.Maybe

phi :: Maybe (Fix Maybe, Bool) -> Bool
phi Nothing = False
phi (Just (a,_)) = isJust $ project a

{-

λ> zygo embed phi (Fix $ Just $ Fix Nothing)
False
λ> para phi (Fix $ Just $ Fix Nothing)
True

-}
```

I think this is the correct implementation.