apple-auth libdef: Allow onPress to return Promise<void>

The example in the doc at
https://docs.expo.io/versions/latest/sdk/apple-authentication/#usage
has this return type for `onPress`, and we want it too. Omitting
`Promise<void>` doesn't seem to be well-considered.

Author: Chris Bobbe

flow-typed/expo-apple-authentication_vx.x.x.js

@@ -40,7 +40,7 @@ declare module 'expo-apple-authentication/build/AppleAuthentication' {
 
 declare module 'expo-apple-authentication/build/AppleAuthentication.types' {
   declare export type AppleAuthenticationButtonProps = {
-    onPress: () => void,
+    onPress: () => void | Promise<void>,
     buttonType: $Values<typeof AppleAuthenticationButtonType>,
     buttonStyle: $Values<typeof AppleAuthenticationButtonStyle>,
     cornerRadius?: number,