Update docs for dockerd.md about `max-concurrent-downloads/max-concurrent-uploads`
Update docs for dockerd.md about `max-concurrent-downloads/max-concurrent-uploads`

It seems that `max-concurrent-downloads` and `max-concurrent-uploads`
are supported in Windows for `config.json`. Though that was not
mentioned in the docs for dockerd.md.

This fix adds the following to the example `config.json` file for Windows:
```
    "max-concurrent-downloads": 3,
    "max-concurrent-uploads": 5,
```

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

Zebrilee/cli,ashfall/cli,ashfall/cli,redpanda/cli,riyazdf/cli,redpanda/cli,thaJeztah/cli,dnephin/cli,e11137/cli,allencloud/cli,thaJeztah/cli,Zebrilee/cli,allencloud/cli,e11137/cli,dnephin/cli,riyazdf/cli,docker/cli,cpuguy83/cli,cpuguy83/cli,docker/cli