message: Fix 0.14 -> 0.15 headline in MIGRATING [ci skip]
diff: @@ -4,7 +4,7 @@ This guide explains what is needed to upgrade contracts when migrating over
  major releases of `cosmwasm`. Note that you can also view the
  [complete CHANGELOG](./CHANGELOG.md) to understand the differences.
  -## 0.14 -> 0.15 (unreleased)
  +## 0.14 -> 0.15
  - Combine `messages` and `submessages` on the `Response` object. The new format
  uses `messages: Vec<SubMsg<T>>`, so copy `submessages` content, and wrap old
  