Fix stale stop command in update guide

Use 'sudo iotedge system stop' instead of 'sudo systemctl stop iotedge'.
The iotedge systemd service was renamed to aziot-edged in IoT Edge 1.2+,
so the old command fails silently on any supported version (1.4/1.5).

Author: jlian

articles/iot-edge/how-to-update-iot-edge.md

@@ -43,7 +43,7 @@ You can [troubleshoot](#troubleshooting) the upgrade process at any time.
 
 When you upgrade between major or minor releases, for example from 1.4 to 1.5, update both the security subsystem and the runtime containers. Before a release, we test the security subsystem and the runtime container version combination. To update between major or minor product releases:
 
-1. On the device, stop IoT Edge using the command `sudo systemctl stop iotedge` and [uninstall](how-to-provision-single-device-linux-symmetric.md#uninstall-iot-edge).
+1. On the device, stop IoT Edge using the command `sudo iotedge system stop` and [uninstall](how-to-provision-single-device-linux-symmetric.md#uninstall-iot-edge).
 
 1. On the device, upgrade your container engine, either [Docker](https://docs.docker.com/engine/install) or [Moby](how-to-provision-single-device-linux-symmetric.md#install-a-container-engine).