[docs] Fix link to createTheme source

Author: oliviertassinari

docs/data/material/customization/default-theme/default-theme.md

@@ -2,7 +2,7 @@
 
 <p class="description">This tree view allows you to explore how the theme object looks like with the default values.</p>
 
-If you want to learn more about how the theme is assembled, take a look at [`material-ui/style/createTheme.js`](https://github.com/mui/material-ui/blob/-/packages/mui-material/src/styles/createTheme.js),
+If you want to learn more about how the theme is assembled, take a look at [`material-ui/style/createTheme.js`](https://github.com/mui/material-ui/blob/-/packages/mui-material/src/styles/createTheme.ts),
 and the related imports which `createTheme` uses.
 
 You can play with the documentation theme object in your browser console,