Fix "manual" ansible-playbook example in guide_vagrant.rst

The given example is trying to load an inventory file with the `-i` option, but somehow the key file name ended up in there instead of the inventory file name. 

I contemplated using `-i vagrant_ansible_inventory_default` instead (which is what my Vagrant demo actually creates) but `-i vagrant_ansible_inventory_machinename` is more consistent with the existing example in the docs.