Fix link to 1.28.0 default report in README (#4232)

Author: bexxmodd

conformance/reports/v1.4.0/istio-istio/README.md

@@ -4,7 +4,7 @@
 
 |API channel|Implementation version|Mode|Report|
 |-----------|----------------------|----|------|
-|x|[1.28.0](https://github.com/istio/istio/releases/tag/1.28.0)|x|[1.28.0 report](./experimental-1.28.0-default-report.yaml)|
+|x|[1.28.0](https://github.com/istio/istio/releases/tag/1.28.0)|x|[1.28.0 report](./1.28.0-default-report.yaml)|
 
 ## Reproduce