docs: remove imports from src in medusa-config.js (#8132)

Importing from `src` in `medusa-config.js` works in development, but would cause an error when running `build`/in production.

This removes from the docs all snippets that imports from the `src` directory. However, we should figure out a better way to handle re-using the module's name.

www/apps/book/app/basics/modules-and-services/page.mdx

@@ -74,7 +74,7 @@ In `medusa-config.js`, add a `modules` property and pass to it your custom modul
 module.exports = defineConfig({
   // ...
   modules: {
-    "helloModuleService": {
+    helloModuleService: {
       resolve: "./modules/hello",
     },
   },