[docs][material-ui] Fix getting started with CSS variables in theme e…

…xample (#43658)

Signed-off-by: Carl Topham <github@carl-topham.com>
Co-authored-by: ZeeshanTamboli <zeeshan.tamboli@gmail.com>

docs/data/material/customization/css-theme-variables/usage.md

@@ -17,7 +17,7 @@ import { ThemeProvider, createTheme } from '@mui/material/styles';
 const theme = createTheme({ cssVariables: true });
 
 function App() {
-  return <ThemeProvider>{/* ...you app */}</ThemeProvider>;
+  return <ThemeProvider theme={theme}>{/* ...your app */}</ThemeProvider>;
 }
 ```