changelog: Simplify task link on sourceforge ingestion

While T735 is the upper level task, that might make curious people follow through too
much indirection. So simplifying to target directly the interesting and up-to-date task.

Related to T3374

docs/archive-changelog.rst

@@ -14,8 +14,8 @@ in this document for historical reasons.
 ----
 
 * **2021-07-22** Completed first archival of SourceForge Git and Subversion
-repositories; regular crawling for those repositories enabled (tracking: `T735
-<https://forge.softwareheritage.org/T735>`__)
+repositories; regular crawling for those repositories enabled (tracking: `T3374
+<https://forge.softwareheritage.org/T3374>`__)
 
 2020
 ----

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3374 is now swh/infra/sysadm-environment#3374 (closed)