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

The given example is trying to load an inventory file with the -i option, so it should probably be given the actual Vagrant-written inventory file name. I contemplated using 'vagrant_ansible_inventory_default' instead (which is what my Vagrant demo actually creates) but 'vagrant_ansible_inventory_machinename' is more consistent with the existing example in the docs.
Merge pull request #5120 from mechfish/patch-1

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