Update device-update-howto-proxy-updates.md

The command of restarting Device Update agent that is mentioned at step#5 is incorrect. It should be "deviceupdate-agent" instead of "adu-agent".

Author: riwaida

articles/iot-hub-device-update/device-update-howto-proxy-updates.md

@@ -87,7 +87,7 @@ This tutorial uses an Ubuntu Server 18.04 LTS virtual machine (VM) as an example
 5. Restart the Device Update agent:
 
    ```sh
-   sudo systemctl restart adu-agent
+   sudo systemctl restart deviceupdate-agent
    ```
 
 ### Set up mock components