Fix name of fixup_backend_upgrade() in changelog (#8959)

Author: msullivan

docs/resources/changelog/6_x.rst

@@ -984,7 +984,9 @@ Fixes
 * Fix UPDATING a link table field with reference to non-link field in some cases
   (:eql:gh:`#8912`)
 
-* Add ``administer rebuild_sql_introspection`` command
+* Add ``administer fixup_backend_upgrade()`` command for repairing
+  certain support views after a separately managed Postgres backend
+  has been upgraded.
   (:eql:gh:`#8926`)
 
 * pg-ext: fix bug in query reparse