Fix type of Easing function to match type in react-native module

Author: Ashoat

src/TypeDefinition.js

@@ -397,7 +397,7 @@ export type NavigationSceneRendererProps = NavigationTransitionProps;
 export type NavigationTransitionSpec = {
   duration?: number,
   // An easing function from `Easing`.
-  easing?: () => any,
+  easing?: (t: number) => number,
   // A timing function such as `Animated.timing`.
   timing?: (value: AnimatedValue, config: any) => any,
 };