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

Summary:
I think in 50667ec 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 #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: #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

Author: buschco

packages/react-native/package.json

@@ -82,7 +82,7 @@
     "gradle.properties",
     "gradle/libs.versions.toml",
     "index.js",
-    "index.flow.js",
+    "index.js.flow",
     "interface.js",
     "jest-preset.js",
     "jest",