fix(billing): update stale comment to match underscore separator

Author: PierreBrisorgueil

modules/billing/controllers/billing.controller.js

@@ -73,7 +73,7 @@ const getUsage = async (req, res) => {
     // Get usage counters (includes month field)
     const usage = await BillingUsageService.get(req.organization._id.toString());
 
-    // Flatten quotas config into { "resource.action": limit } format
+    // Flatten quotas config into { "resource_action": limit } format
     // Normalize Infinity to null for JSON-safe serialization
     const quotas = config.billing?.quotas;
     let limits = {};