Update readme

Author: PeterTYLiu

README.md

@@ -2,7 +2,7 @@
 
 Live app: https://petertyliu.github.io/mui-theme-builder/
 
-Use this app to build/generate a basic MUI theme, which you can customize further by referring to the [MUI theming docs](https://mui.com/material-ui/customization/theming). Built with MUI 7, Compatible with MUI 5 and higher.
+Use this app to build/generate a basic MUI theme, which you can customize further by referring to the [MUI theming docs](https://mui.com/material-ui/customization/theming). Built with MUI 9, Compatible with MUI 5 and higher.
 
 Inspired by (but not forked from) [bareynol's MUI theme creator](https://bareynol.github.io/mui-theme-creator)