docs: Fix sphinx warning
This fixes the build [1] [2]
[1]
10:49:23 Warning, treated as error: 10:49:23 /var/lib/jenkins/workspace/DDEP/tests-on-diff/docs/README.rst:40:hardcoded link 'https://archive.softwareheritage.org/save/' could be replaced by an extlink (try using ':swh_web:`save/`' instead)
[2] https://jenkins.softwareheritage.org/view/swh-draft/job/DDEP/job/tests/1741/console