Fix label selector in Using Source IP tutorial
Before:
```sh
controlplane $ kubectl delete svc -l run=source-ip-app
No resources found
```
```sh
k get svc --show-labels
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE LABELS
clusterip ClusterIP 10.110.247.112 <none> 80/TCP 28m app=source-ip-app
kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 49m component=apiserver,provider=kubernetes
loadbalancer LoadBalancer 10.111.123.206 <pending> 80:31111/TCP 9m59s app=source-ip-app
nodeport NodePort 10.108.129.31 <none> 80:32503/TCP 22m app=source-ip-app
```
After:
```sh
controlplane $ kubectl delete svc -l app=source-ip-app
service "clusterip" deleted
service "loadbalancer" deleted
service "nodeport" deleted
```
Update source-ip.md
Update source-ip.md
Update source-ip.md
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