Outdated reference to InkSplashType

https://docs.flutter.io/flutter/material/InkRipple-class.html

```
This object is rarely created directly. Instead of creating an ink ripple, consider using an InkResponse or InkWell widget, which uses gestures (such as tap and long-press) to trigger ink splashes. This class is used when the Theme's ThemeData.splashType is InkSplashType.ripple.
```

This doesn't appear to exist anymore.