Fix path to react-native for Metro (#52877)

Summary:
Pull Request resolved: #52877

We moved the HelloWorld package from `packages`to `private` but we didn't update the path to React Native that is consumed by metro.

This change fixes this.

## Changelog:
[Internal] -

Reviewed By: huntie

Differential Revision: D78493478

fbshipit-source-id: 8795c2963dec036693a73a16c18be381f19e11c4

Author: cipolleschi

private/helloworld/scripts/metro.js

@@ -23,7 +23,7 @@ program
           android: {},
         },
         root: path.join(__dirname, '../'),
-        reactNativePath: path.join(__dirname, '../../react-native'),
+        reactNativePath: path.join(__dirname, '../../../packages/react-native'),
       },
       {
         experimentalDebugger: false,