[Tiny] Fixed incorrect import in react-server-dom-webpack (#25554)

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## Summary

In #25504,
`react-server-dom-webpack/` was deprecated in favor of
`react-server-dom-webpack/client`, but a remaining import wasn't
adjusted accordingly.

As a result, the remaining conditions within the file are no longer
firing appropriately, which I ran into while playing around with a fork
of
[server-components-demo](https://github.com/reactjs/server-components-demo).

The `index.js` file now contains a
[placeholder](https://github.com/facebook/react/blob/main/packages/react-server-dom-webpack/index.js)
and the actual logic of the client now sits in `/client`.

## How did you test this change?

I replaced `require.resolve('../')` with `require.resolve('../client')`
in the `react-server-dom-webpack` package in `node_modules` and
confirmed that the output of the build looked good again.

packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js

@@ -40,7 +40,7 @@ class ClientReferenceDependency extends ModuleDependency {
 // without the client runtime so it's the first time in the loading sequence
 // you might want them.
 const clientImportName = 'react-server-dom-webpack/client';
-const clientFileName = require.resolve('../');
+const clientFileName = require.resolve('../client');
 
 type ClientReferenceSearchPath = {
   directory: string,