Clarify rs-default-mr-target changelog

Previously it sounded like we'd always target the fork, regardless of
where the MR was opened.

changelogs/unreleased/rs-default-mr-target.yml

 ---
-title: 'When a forked project is less visible than its source, merge requests now target the less visible project by default.'
+title: 'When a forked project is less visible than its source, merge requests opened in the fork now target the less visible project by default.'
 merge_request: 21517
 author:
 type: changed