const impl formatting doesn't work

It seems the const impl syntax isn't being pasrsed!

```
thread '<unnamed>' (78880) panicked at ...\index.crates.io-1949cf8c6b5b557f\prettyplease-0.2.37\src\item.rs:364:13:
  not implemented: Item::Verbatim `impl const std :: ops :: Deref for cLockableTexture < > { type Target = crate :: bindings :: Hw :: Graphics :: cTexture < > ; fn deref (& self) -> & Self :: Target { & self . base } }`
```