Replace link to Vagrant Cloud name

This is just an example, you can create box by you self. 

My box points directly to https://raw.githubusercontent.com/smdahlen/vagrant-digitalocean/master/box/digital_ocean.box so it's always up-to-date with your repo.

This should be carefully tested before merge :imp: 

P.S.: I had issues with adding box as link, also more kosher to create Vagrant file by calling:
```bash
vagrant init AndrewDryga/digital-ocean
```
:)