Add #18019 to the CHANGELOG.

CHANGELOG

@@ -16,6 +16,7 @@ v 8.9.0 (unreleased)
   - Add DB index on users.state
   - Add rake task 'gitlab:db:configure' for conditionally seeding or migrating the database
   - Changed the Slack build message to use the singular duration if necessary (Aran Koning)
+  - Links from a wiki page to other wiki pages should be rewritten as expected
   - Fix issues filter when ordering by milestone
   - Todos will display target state if issuable target is 'Closed' or 'Merged'
   - Fix bug when sorting issues by milestone due date and filtering by two or more labels