Use bento prefix for box names in README

[leejones]

First up, thanks for bento! I've used these boxes for a while now and really appreciate the effort that goes into this project ✨.

I ran into a small issue, but I think I have a solution. vagrant up failed when I used the chef/ prefix in my Vagrantfile.

Here is the error I experienced:

$> vagrant --version
Vagrant 1.7.4
$> cat Vagrantfile
Vagrant.configure(2) do |config|
  config.vm.box = "chef/ubuntu-12.04"
end
$> vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'chef/ubuntu-12.04' could not be found. Attempting to find and install...
    default: Box Provider: virtualbox
    default: Box Version: >= 0
==> default: Loading metadata for box 'chef/ubuntu-12.04'
    default: URL: https://atlas.hashicorp.com/chef/ubuntu-12.04
The box you're adding has a name different from the name you
requested. For boxes with metadata, you cannot override the name.
If you're adding a box using `vagrant box add`, don't specify
the `--name` parameter. If the box is being added via a Vagrantfile,
change the `config.vm.box` value to match the name below.

Requested name: chef/ubuntu-12.04
Actual name: bento/ubuntu-12.04

I updated the README based on the recommendation in the error message. The bento/ prefix works in my testing for both vagrant up and vagrant box add.

vagrant box add works with the chef/ prefix because it appears to redirect from the chef/ prefix to the bento/ one. I updated the vagrant box add example to use the bento/ prefix since it seems to be the new location and for consistency between the two examples.

[cheeseplus]

Just missed this when we were updating - +1