publish `index.js.flow` instead of `index.flow.js` (#52179)

Summary:
I think in https://github.com/facebook/react-native/commit/50667eceb1be4771375d6a3cc2f4e42d4d8aad3a the file name was mixed up. Instead of `index.flow.js` it should be `index.js.flow` (see https://github.com/facebook/react-native/blob/main/packages/react-native/index.js.flow)

Should fix https://github.com/facebook/react-native/issues/51885

## Changelog:

<!-- Help reviewers and the release process by writing your own changelog entry.

[General][Added] Publish top-level Flow types for `react-native`

For more details, see:
https://reactnative.dev/contributing/changelogs-in-pull-requests

Pull Request resolved: https://github.com/facebook/react-native/pull/52179

Test Plan:
Steps to reproduce

1. Setup a react-native project with flow (or clone https://github.com/buschco/react-native-flow + `cd ReproducerApp && npm i`)
2. `import {Alert} from 'react-native'` -> `Alert` has type `any`
3. add `index.js.flow` File (https://github.com/facebook/react-native/blob/v0.79.3/packages/react-native/index.js.flow) to `node_modules/react-native/index.js.flow` -> `Alert` no longer `any`

Reviewed By: christophpurrer

Differential Revision: D77052871

Pulled By: robhogan

fbshipit-source-id: 32b0052a9d96486aff66a1f6e4577ff62cbcd97e