doc: add 8.4.0 link to CHANGELOG.md

PR-URL: https://github.com/nodejs/node/pull/15064
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

CHANGELOG.md

@@ -27,7 +27,8 @@ release.
 </tr>
 <tr>
     <td valign="top">
-<b><a href="doc/changelogs/CHANGELOG_V8.md#8.3.0">8.3.0</a></b><br/>
+<b><a href="doc/changelogs/CHANGELOG_V8.md#8.4.0">8.4.0</a></b><br/>
+<a href="doc/changelogs/CHANGELOG_V8.md#8.3.0">8.3.0</a><br/>
 <a href="doc/changelogs/CHANGELOG_V8.md#8.2.1">8.2.1</a><br/>
 <a href="doc/changelogs/CHANGELOG_V8.md#8.2.0">8.2.0</a><br/>
 <a href="doc/changelogs/CHANGELOG_V8.md#8.1.4">8.1.4</a><br/>