Fix the command vagrant box add in README.md

sicness@abalashov-nt /tmp $ vagrant box init precise64 http://bit.ly/vagrant-lxc-precise64-2013-10-23
Usage: vagrant box <command> [<args>]

Available subcommands:
     add
     list
     remove
     repackage

For help on any individual command run `vagrant box COMMAND -h`
sicness@abalashov-nt /tmp $ vagrant init precise64 http://bit.ly/vagrant-lxc-precise64-2013-10-23
A `Vagrantfile` has been placed in this directory. You are now
ready to `vagrant up` your first virtual environment! Please read
the comments in the Vagrantfile as well as documentation on
`vagrantup.com` for more information on using Vagrant.