Update connecting-frontend-backend.md

```
Error from server (NotFound): deployments.apps "hello" not found
```
chang hello to backend

```
Name:                   backend
Namespace:              default
CreationTimestamp:      Sun, 18 Jul 2021 07:33:39 +0800
Labels:                 <none>
```

content/zh/docs/tasks/access-application-cluster/connecting-frontend-backend.md

@@ -82,7 +82,7 @@ View information about the backend Deployment:
 查看后端的 Deployment 信息：
 
 ```shell
-kubectl describe deployment hello
+kubectl describe deployment backend
 ```
 
 <!--