Fix test comment typo (#13568)

Author: NE-SmallTown

packages/react-dom/src/__tests__/ReactMount-test.js

@@ -302,7 +302,7 @@ describe('ReactMount', () => {
       ReactDOM.render(<Foo>a</Foo>, container2);
       // The update did not flush yet.
       expect(container1.textContent).toEqual('1');
-      // The initial mount flushed, but not the update scheduled in cDU.
+      // The initial mount flushed, but not the update scheduled in cDM.
       expect(container2.textContent).toEqual('a');
     });
     // All updates have flushed.