docs: Update archive changelog about the foss.heptpod.net instance

Related to T3581

docs/archive-changelog.rst

@@ -14,13 +14,18 @@ in this document for historical reasons.
 2021
 ----
 
+* **2021-09-22** Completed first archival of the `Heptapod GitLab instance
+  <https://foss.heptapod.net>_`. This is the first forge with mostly mercurial origins.
+  Regular crawling for those repositories enabled (tracking: `T3581
+  <https://forge.softwareheritage.org/T3581>`_)
+
 * **2021-08-03** Completed first archival of SourceForge Mercurial repositories; regular
   crawling for those repositories enabled (tracking: `T3374
-  <https://forge.softwareheritage.org/T3374>`__)
+  <https://forge.softwareheritage.org/T3374>`_)
 
 * **2021-07-22** Completed first archival of SourceForge Git and Subversion
   repositories; regular crawling for those repositories enabled (tracking:
-  `T3374 <https://forge.softwareheritage.org/T3374>`__)
+  `T3374 <https://forge.softwareheritage.org/T3374>`_)
 
 
 2020