fix(billing): use default import for billingEvents in README example

Author: PierreBrisorgueil

modules/billing/README.md

@@ -53,7 +53,7 @@ await BillingUsageService.increment(organizationId, 'documents.create', 1);
 Listen for plan changes in downstream modules:
 
 ```js
-import { billingEvents } from '../billing/lib/events.js';
+import billingEvents from '../billing/lib/events.js';
 
 billingEvents.on('plan.changed', ({ organizationId, previousPlan, newPlan, isDowngrade }) => {
   if (isDowngrade) {