doc: add git node security --cleanup

PR-URL: nodejs#54381
Reviewed-By: Ruy Adorno <ruy@vlt.sh>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

doc/contributing/security-release-process.md

@@ -159,8 +159,9 @@ security announcements for more information.
     folder and merging the PR.
 
 * [ ] 2\. **Cleanup:**
-  * [ ] Close PRs and backports.
-  * [ ] Close HackerOne reports:
+  * [ ] `git node security --cleanup`. This command will:
+  * Close PRs and backports.
+  * Close HackerOne reports:
     * Close Resolved
     * Request Disclosure
     * Request publication of H1 CVE requests