docs: fix markdown lint issues in command docs

cmd/serve/docker/docker.md

@@ -8,7 +8,8 @@ docker daemon and runs the corresponding code when necessary.
 Docker plugins can run as a managed plugin under control of the docker daemon
 or as an independent native service. For testing, you can just run it directly
 from the command line, for example:
-```
+
+```sh
 sudo rclone serve docker --base-dir /tmp/rclone-volumes --socket-addr localhost:8787 -vv
 ```