docs: update broken documentation example (#650)

update documentation example to correctly use changes from PR #641

Author: IanGallacher

docs/usage.md

@@ -30,7 +30,7 @@ import { sveltePreprocess } from 'svelte-preprocess';
  * @type {import('@sveltejs/kit').Config}
  */
 const config = {
-  preprocess: preprocess({
+  preprocess: sveltePreprocess({
     // ...svelte-preprocess options
   }),
   // ...other svelte options
@@ -140,4 +140,4 @@ export default {
     ],
   },
 };
-```
+```