Edits to "Installing Puppet" section

* Better indicate that we're working with the new instances created in the previous section.
* Fix parallelism in the first bullet point.
* Add "to" left out of the second bullet point.
* As with the other bullet points, note in the first bullet point why --install-script is required.
* In the third bullet point, clarify that we're referring to usernames
* Clarify the nature of the puppet node install argument.
* Tighten language.
* Remove redundant spaces for consistency.