docs: change es6 import to cjs require

Author: ignatiusmb

Diff for: docs/usage.md

@@ -107,7 +107,7 @@ export default {
 
 ```js
 // svelte.config.js
-import sveltePreprocess from 'svelte-preprocess';
+const sveltePreprocess = require('svelte-preprocess');
 
 module.exports = {
   preprocess: sveltePreprocess({