Change version number of dev version from 0.18 to 0.17 in release notes

When browsing the docs I stumbled over this small inconsistency in the release notes: in the release notes, the version number of the next (unreleased) version is 0.18, but 0.17 is missing. I think it should be 0.17?

Sorry for this really minor pull request, I don't mean to be annoying!