Update archive changelog about the opam.ocaml.org instance

Related to T3568

docs/archive-changelog.rst

@@ -14,6 +14,10 @@ in this document for historical reasons.
 2021
 ----
 
+* **2021-09-25** Completed first archival of the `Opam repository
+  <https://opam.ocaml.org>_`. Regular crawling for those repositories
+  enabled (tracking: `T3424 <https://forge.softwareheritage.org/T3424>`_)
+
 * **2021-09-23** Completed first archival of the `Heptapod instance
   <https://forge.extranet.logilab.fr/>_`. Regular crawling for those repositories
   enabled (tracking: `T3597 <https://forge.softwareheritage.org/T3597>`_)

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3568 is now swh/infra/sysadm-environment#3568 (closed)