docs: tighten defaultRegion changeset wording

Author: nicktrn

.changeset/project-default-region-response.md

@@ -2,4 +2,4 @@
 "@trigger.dev/core": patch
 ---
 
-Add `defaultRegion` (the project's default worker-group name, or null when unset) to the project GET and list API responses.
+Add `defaultRegion` to the project GET and list API responses; null when unset.