Fix sample data in example Cost Insights client

Signed-off-by: Tim Hansen <timbonicus@gmail.com>

.changeset/cost-insights-two-crabs-evolve.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-cost-insights': patch
+---
+
+Fixed generation of sample data in the example Cost Insights client

plugins/cost-insights/src/testUtils/testUtils.ts

@@ -61,7 +61,7 @@ export function aggregationFor(
   const days = DateTime.fromISO(endDate).diff(
     DateTime.fromISO(inclusiveStartDateOf(duration, inclusiveEndDate)),
     'days',
-  );
+  ).days;
 
   function nextDelta(): number {
     const varianceFromBaseline = 0.15;