fix walkthrough example

The service created by this command did not match the port name used
later on in the walkthrough. Therefor it is defined explicitly here.

docs/walkthrough.md

@@ -94,9 +94,33 @@ spec:
 
 </details>
 
+<details>
+
+<summary>sample-app.service.yaml</summary>
+
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  labels:
+    app: sample-app
+  name: sample-app
+spec:
+  ports:
+  - name: http
+    port: 80
+    protocol: TCP
+    targetPort: 8080
+  selector:
+    app: sample-app
+  type: ClusterIP
+```
+
+</details>
+
 ```shell
 $ kubectl create -f sample-app.deploy.yaml
-$ kubectl create service clusterip sample-app --tcp=80:8080
+$ kubectl create -f sample-app.service.yaml
 ```
 
 Now, check your app, which exposes metrics and counts the number of