correct step 7 infra.psf.io migration guide

[cegerhardson]

A small change should be made to infra.psf.io's Migrating to a new host guide, step 7 of "Shutdown and reclaim hostname" to include salt-minion to the command. Step should look like:

7. Restart the salt minion user@new-host:~$ sudo salt-call service.restart salt-minion