message: use diff codeblock in changeset markdown
diff: 2. The `LocalPublish` publisher can now be created using `fromConfig`:
  -```
  ---- const publisher = new LocalPublish(config, logger, discovery);
  -+++ const publisher = LocalPublish.fromConfig(config, logger, discovery);
  +```diff
  +- const publisher = new LocalPublish(config, logger, discovery);
  ++ const publisher = LocalPublish.fromConfig(config, logger, discovery);
  ```
  