[Docs] typo in ecg_quality docstring

[atpage]

Description

'than' -> 'then'

Checklist

• I have read the CONTRIBUTING file.
• My PR is targeted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)

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Thanks for opening this pull request! We'll make sure it's perfect before merging 🤗
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[DominiqueMakowski]

good catch, thanks!

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Congrats on merging your first pull request! 🎉🍾 We're looking forward to your next one!