Fix migration to change volunteer acceptance values

https://trello.com/c/eWaGN79r/125-bug-falscher-fw-status-auf-staging

I made a mistake in this migration which caused all volunteers except undecided to end up with the state resigned.

To "fix" this locally run Volunteer.where(acceptance: 4).update_all(acceptance: 2) in the Rails console. I did the same on staging, and on production we fortunately didn't run this migration yet.