change onRefresh flow typing (#28167)

Summary:
I propose this change because we (and a lot of other people, I'd guess) pass an `async` function as a parameter to `onRefresh`. Because the `async` function returns a `promise`, flow is reporting an error. I think the type checking here can be relaxed either all the way to `any` (because RN does not care here what we return) or to `void | Promise<void>` to account for async functions.

looking at fb7b2d3#diff-a9c5687ae65236ba3e7f34bfdcdec81d seems like the second is preferred

## Changelog

[General] [changed] - relax RefreshControl's onRefresh flow typing
Pull Request resolved: #28167

Test Plan: * flow passes

Reviewed By: hramos

Differential Revision: D20196529

Pulled By: TheSavior

fbshipit-source-id: bb5a314bcfb5fb9c8ab71eccb449f1322aeebacb

Libraries/Components/RefreshControl/RefreshControl.js

@@ -83,7 +83,7 @@ export type RefreshControlProps = $ReadOnly<{|
   /**
    * Called when the view starts refreshing.
    */
-  onRefresh?: ?() => void,
+  onRefresh?: ?() => void | Promise<void>,
 
   /**
    * Whether the view should be indicating an active refresh.