Update getting-started.md

Currently it is necessary to provide valid 'theme' and 'darkTheme' props to <HydraAdmin> or it fails with no useful error message.

- https://stackoverflow.com/questions/77840771/error-while-generate-an-admin-interface-using-api-platform-admin-and-react-admin

- marmelab/react-admin#9687

admin/getting-started.md

@@ -27,10 +27,15 @@ For instance, if you used Create React App, replace the content of `src/App.js`
 ```javascript
 import { HydraAdmin } from "@api-platform/admin";
 
+import { defaultTheme } from 'react-admin';
+
+const lightTheme = defaultTheme;
+const darkTheme = { ...defaultTheme, palette: { mode: 'dark' } };
+
 // Replace with your own API entrypoint
 // For instance if https://example.com/api/books is the path to the collection of book resources, then the entrypoint is https://example.com/api
 export default () => (
-  <HydraAdmin entrypoint="https://demo.api-platform.com" />
+  <HydraAdmin entrypoint="https://demo.api-platform.com" theme={lightTheme} darkTheme={darkTheme}/>
 );
 ```