apertis-pkg-merge-local: Improve changelog

In order to allow developers to summarize Apertis specific changes add
an additional placeholder.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

tools/apertis-pkg-merge-local

@@ -98,7 +98,9 @@ release-down() {
 }
 
 release-up() {
-    dch --force-distribution --distribution apertis --local +apertis --upstream Merge changes from $UPSTREAM_BRANCH
+    dch --local +apertis --upstream Merge updates from $UPSTREAM_BRANCH
+    dch Remaining Apertis specific changes:
+    dch --force-distribution --distribution apertis
     release
 }