fix CssBaseline import in example code

First code example on page uses
  `import { ThemeProvider, createTheme, CssBaseline } from '@mui/material/styles';`
which fails with
  `export 'CssBaseline' (imported as 'CssBaseline') was not found in '@mui/material/styles' ... `.

Presumably, `CssBaseline` was moved recently and the first code example was not updated correspondingly; last code example on page is fine.

docs/data/material/customization/dark-mode/dark-mode.md

@@ -7,7 +7,8 @@
 You can make your application use the dark theme as the default—regardless of the user's preference—by adding `mode: 'dark'` to the `createTheme` helper:
 
 ```js
-import { ThemeProvider, createTheme, CssBaseline } from '@mui/material/styles';
+import { ThemeProvider, createTheme } from '@mui/material/styles';
+import CssBaseline from '@mui/material/CssBaseline';
 
 const darkTheme = createTheme({
   palette: {