Update README.md to fix Vagrant box names in example

[mleert]

Minor update to the README to use the same Vagrant box naming format as the Packer JSON.

Comes from the box outputs here:

• i686 - https://github.com/nix-community/nixbox/blob/97c04c470f59b00738915f9da915b0e4f165c7e0/nixos-i686.json#L136
• x86_64 - https://github.com/nix-community/nixbox/blob/97c04c470f59b00738915f9da915b0e4f165c7e0/nixos-x86_64.json#L136