rename mobilenet-ssd-512-0001.params

better to name it to `ssd_mobilenet_512-0000.params`
- use underscores `_`
- use `0000`
- use` ssd_` prefix

if it is renamed it will be easier to use it with `deploy.py`
```
python deploy.py --network mobilenet --data-shape 512
```

https://github.com/zhreshold/mxnet-ssd/releases/download/v0.7-alpha/mobilenet-ssd-512.zip