[demo] add namespace to kubectl instructions (#2611)

Co-authored-by: Juliano Costa <[email protected]>
Co-authored-by: Phillip Carter <[email protected]>

content/en/docs/demo/kubernetes-deployment.md

@@ -41,7 +41,8 @@ The following command will install the demo application to your Kubernetes
 cluster.
 
 ```shell
-kubectl apply -f https://raw.githubusercontent.com/open-telemetry/opentelemetry-demo/main/kubernetes/opentelemetry-demo.yaml
+kubectl create namespace otel-demo
+kubectl apply --namespace otel-demo -f https://raw.githubusercontent.com/open-telemetry/opentelemetry-demo/main/kubernetes/opentelemetry-demo.yaml
 ```
 
 > **Note** These manifests are generated from the Helm chart and are provided