docs: update host-gateway-ip to use daemon.json instead of cli flag

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

Author: dvdksn

docs/reference/commandline/dockerd.md

@@ -849,7 +849,9 @@ flag for the dockerd command line interface, or the `host-gateway-ip` key in
 the daemon configuration file.
 
 ```console
-$ dockerd --host-gateway-ip 192.0.2.0
+$ cat > /etc/docker/daemon.json
+{ "host-gateway-ip": "192.0.2.0" }
+$ sudo systemctl restart docker
 $ docker run -it --add-host host.docker.internal:host-gateway \
   busybox ping host.docker.internal 
 PING host.docker.internal (192.0.2.0): 56 data bytes