Update Fade.d.ts to fix incorrect onLeave typing

## Motivation

When testing, we realized that our `onLeave` callback wasn't being called at all for the `Fade` component.

Looking at the types in react-transition-group, it looks like the actual correct prop is `onExit`.

## Fix

Update typings to `onExit`

types/lib/Fade.d.ts

@@ -14,7 +14,7 @@ export interface FadeProps extends React.HTMLAttributes<HTMLElement> {
   transitionAppear?: boolean;
   transitionEnter?: boolean;
   transitionLeave?: boolean;
-  onLeave?: () => void;
+  onExit?: () => void;
   onEnter?: () => void;
   innerRef?: React.Ref<HTMLElement>;
 }