give instructions for material ui 5, not 4 (rjsf-team#4126)

Author: burnettk

packages/docs/docs/usage/themes.md

@@ -40,10 +40,10 @@ For example, to use the standard bootstrap 3 form, you can run:
 import Form from '@rjsf/core';
 ```
 
-To use the material-ui form, you should first install both `@rjsf/core` and `@rjsf/material-ui`. Then, you can run:
+To use the material-ui 5 form, you should first install both `@rjsf/core` and `@rjsf/mui`. Then, you can run:
 
 ```ts
-import Form from '@rjsf/material-ui';
+import Form from '@rjsf/mui';
 ```
 
 For more information on how to create a custom theme, see documentation on the `withTheme` component.