Localization: Fix "Find the source" link

Author: ValentinH

docs/src/pages/guides/localization/localization.md

@@ -59,7 +59,7 @@ const theme = createMuiTheme({
 | Ukrainian | uk-UA | `ukUA` |
 | Vietnamese | vi-VN | `viVN` |
 
-You can [find the source](https://github.com/mui-org/material-ui/blob/master/packages/material-ui/src/locale/index.js) in the GitHub repository.
+You can [find the source](https://github.com/mui-org/material-ui/blob/master/packages/material-ui/src/locale/index.ts) in the GitHub repository.
 
 To create your own translation, or to customise the English text, copy this file to your project, make any changes needed and import the locale from there.