message: Fix upgrade guide header
diff: This document describes breaking changes and how to upgrade. For a complete list of changes including minor and patch releases, please refer to the [changelog](CHANGELOG.md).
  -## v5.0.0-rc1
  +## v5
  +
  +_Currently available as prerelease: `npm i leveldown@next`._
  This is a rewrite to N-API - which is a huge milestone, achieved without an impact on write performance - and an upgrade to `abstract-leveldown` v6, which solves long-standing issues around serialization and type support.
  