Fix change index version of 3.9.0 to 84

The change index version was reverted back from 85 to 84 with
9339e8563c Revert "Bump Lucene version to 9.7.0".

See I98122ca165d5b7b2e2d40a45b35a52496abd8c80

Change-Id: I344bbc95cb6e93699625d52b59e5a2cec3e499e4

pages/site/releases/3.9.md

diff --git a/pages/site/releases/3.9.md b/pages/site/releases/3.9.md
index 6c519d4..b013ff9 100644
--- a/pages/site/releases/3.9.md
+++ b/pages/site/releases/3.9.md
@@ -136,7 +136,7 @@
 This release doesn't contain schema changes.
 
 The `accounts` index version has been increased to version _13_.
-The `changes` index version has been increased to version _85_.
+The `changes` index version has been increased to version _84_.
 The `groups` index version has been increased to version _10_.
 The `projects` index version has been increased to version _8_.