Add svelte file extension to tailwind puge configuration. (#964)

Self explanatory.

docs/src/pages/guides/styling.md

@@ -128,7 +128,7 @@ And also create a `tailwind.config.js` in your project root:
 // tailwind.config.js
 module.exports = {
   mode: 'jit',
-  purge: ['./public/**/*.html', './src/**/*.{astro,js,jsx,ts,tsx,vue}'],
+  purge: ['./public/**/*.html', './src/**/*.{astro,js,jsx,svelte,ts,tsx,vue}'],
   // more options here
 };
 ```