update Sphinx extension submodule reference
minor #4394 update Sphinx extension submodule reference (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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update Sphinx extension submodule reference

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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5ca2df8 update Sphinx extension submodule reference