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.

docsite/rst/guide_vagrant.rst

@@ -117,7 +117,7 @@ Here is an example:
 
 .. code-block:: bash
    
-    $ ansible-playbook -i insecure_private_key --private-key=~/.vagrant.d/insecure_private_key -u vagrant playbook.yml
+    $ ansible-playbook -i vagrant_ansible_inventory_machinename --private-key=~/.vagrant.d/insecure_private_key -u vagrant playbook.yml
 
 .. seealso::