fix chapter 4 timestamp check matcher

Author: lucasfcosta

chapter3/2_writing_good_assertions/4_manual_assertions/inventoryController.test.js

@@ -4,5 +4,5 @@ test("generatedAt in the past", () => {
   const result = getInventory();
   const currentTime = Date.now() + 1;
   const isPastTimestamp = result.generatedAt.getTime() <= currentTime;
-  expect(isPastTimestamp).toBeBefore(true);
+  expect(isPastTimestamp).toBe(true);
 });