Make a distinction between 'edit' and 'reword' commands.

[ashleyharvey]

As I was reading your book, it wasn't immediately clear what the difference between git-rebase's edit and reword commands are. A quick Google gave me an answer, so I'll submit a PR on this issue.

Thanks for your guide, by the way, I find it much easier to understand than most of the content out there on this subject.