[Fix] Fixed rodrigues2021 ECG peaks detector (make it work with < 50Hz sampling rate)

[LucaCerina]

Description

This PR aims at fixing a bug where 'Nd' variable in _ecg_findpeaks_rodrigues was set to 0 in case of sampling_rate too small, effectively nulling the ECG first derivative

Proposed Changes

I have set N to be at least 2, so that Nd is at least 1.

Checklist

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• 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)

[welcome]

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

Nice thanks!

[welcome]

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