doc: add missing changelog heading for 7.7.2

When the release was cut, the changelog heading in CHANGELOG_V7
was accidentally omitted.

PR-URL: nodejs#11823
Fixes: nodejs#11822
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/changelogs/CHANGELOG_V7.md

@@ -6,6 +6,7 @@
 </tr>
 <tr>
 <td>
+<a href="#7.7.2">7.7.2</a><br/>
 <a href="#7.7.1">7.7.1</a><br/>
 <a href="#7.7.0">7.7.0</a><br/>
 <a href="#7.6.0">7.6.0</a><br/>