Update README.md

Yup. Had to dive into the source to figure this one out.
Merge pull request #25 from dafyddcrosby/patch-1

Update README.md to show how to add per-machine provisioner options
Fix typo 'pey-pair-name' -> 'key-pair-name'
Merge pull request #30 from dafyddcrosby/patch-2

Fix typo 'pey-pair-name' -> 'key-pair-name'
Merge branch 'master' of https://github.com/opscode/chef-metal