unstable_createRoot -> createRoot in test

Fixes test added in facebook#17105, which was based on an earler commit than the
one that removed the `unstable_` prefix from `createRoot`.

Author: acdlite

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -2526,7 +2526,7 @@ describe('ReactDOMServerPartialHydration', () => {
     container.innerHTML = finalHTML;
 
     suspend = true;
-    let root = ReactDOM.unstable_createRoot(container, {hydrate: true});
+    let root = ReactDOM.createRoot(container, {hydrate: true});
     root.render(<App showSibling={false} />);
     expect(Scheduler).toFlushAndYield([]);