cookers/base: Fix sphinx >= 4.4 warning

Fix more than one target found for cross-reference 'ObjectType' sphinx warning.

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Migrated from D7762 (view on Phabricator)

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Patch application report for D7762 (id=28081)

Rebasing onto 5348fcd4...

Current branch diff-target is up to date.

Changes applied before test

commit 835b34f142bae5669e08e213ae4a02e491544804
Author: Antoine Lambert <anlambert@softwareheritage.org>
Date:   Fri May 6 17:03:22 2022 +0200

    cookers/base: Fix sphinx >= 4.4 warning
    
    Fix "more than one target found for cross-reference 'ObjectType'"
    sphinx warning.

See https://jenkins.softwareheritage.org/job/DVAU/job/tests-on-diff/230/ for more details.

mentioned in merge request swh-docs!225 (closed)

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