Update README.md

Fix kubectl commands

Author: biefy

Diff for: examples/istio-based-service-mesh/observability/README.md

@@ -9,8 +9,8 @@ Based on artifacts from the open-source community, this directory contains sampl
 ### Install
 
 ```shell
-kubectl apply prometheus.yaml
-kubectl apply grafana.yaml
+kubectl apply -f prometheus.yaml
+kubectl apply -f grafana.yaml
 ```
 
 ### Test Grafana
@@ -19,4 +19,4 @@ kubectl apply grafana.yaml
 kubectl -n aks-istio-system port-forward service/grafana 3000:3000
 ```
 
-Open http://localhost:3000 in your browser to monitor the health status of your mesh.
+Open http://localhost:3000 in your browser to monitor the health status of your mesh.