Updated docker image paths to nsqio/*

Updated docker image paths to `nsqio/*`
@pilwon I simply mean we need to update this suggested command line here, in this document:

```
docker run --name nsqd -p 4150:4150 -p 4151:4151 \
    nsqio/nsqd \
    --broadcast-address=my.public.host.ip \
    --lookupd-address=<host>:<port>
```