doc: add v15.4.0 link to CHANGELOG.md

PR-URL: #36456
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>

CHANGELOG.md

@@ -32,7 +32,8 @@ release.
 </tr>
 <tr>
     <td valign="top">
-<b><a href="doc/changelogs/CHANGELOG_V15.md#15.3.0">15.3.0</a></b><br/>
+<b><a href="doc/changelogs/CHANGELOG_V15.md#15.4.0">15.4.0</a></b><br/>
+<a href="doc/changelogs/CHANGELOG_V15.md#15.3.0">15.3.0</a><br/>
 <a href="doc/changelogs/CHANGELOG_V15.md#15.2.1">15.2.1</a><br/>
 <a href="doc/changelogs/CHANGELOG_V15.md#15.2.0">15.2.0</a><br/>
 <a href="doc/changelogs/CHANGELOG_V15.md#15.1.0">15.1.0</a><br/>