[docs] Fix missing import in auto-dark theme palette example (#19694)

Author: queengooborg

docs/src/pages/customization/palette/palette.md

@@ -146,7 +146,7 @@ For instance, you can enable the dark mode automatically:
 ```jsx
 import React from 'react';
 import useMediaQuery from '@material-ui/core/useMediaQuery';
-import { ThemeProvider } from '@material-ui/core/styles';
+import { createMuiTheme, ThemeProvider } from '@material-ui/core/styles';
 
 function App() {
   const prefersDarkMode = useMediaQuery('(prefers-color-scheme: dark)');