Avoid warning in documentation build

[bouweandela]

Description

Avoid a warning in the documentation build introduced in #3628.

• Link to documentation: https://esmvaltool--3675.org.readthedocs.build/en/3675/

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• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🛠 Documentation is available
• [x] 🛠 Tests run successfully
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

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