some (mostly) minor edits and proofreading changes

Having followed the tutorial now from start to finish, I have some suggested edits, mostly proofreading-type correction of typos and wording that sounded a little odd to a native english speaker.  I also removed the reference to the db error at the end of chapter 12 (Deploy), as suggested by @bmispelon. I did not encounter any error, probably due to the fact that the deploy chapter has been moved further forward in the tutorial.