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: https://github.com/nodejs/node/pull/11823
Fixes: https://github.com/nodejs/node/issues/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>