Update the order of release note page

In order to support automatically updating the release notes when we
create stable branches, we want the pages to be in a standard order.
This patch updates the order to be reverse chronological, so the most
recent notes appear at the top.

TrivialFix.

Change-Id: I8eca97a75f8c6ee1f41712678b12b7e9dac655dd

releasenotes/source/index.rst

@@ -7,9 +7,9 @@ Contents
 .. toctree::
    :maxdepth: 2
 
-   liberty
-   mitaka
    unreleased
+   mitaka
+   liberty
 
 
 Indices and tables