Fix Thielen2021 paper DOI pointing to wrong publication

[Copilot]

The Thielen2021 dataset had its doi set to the dataset repository DOI (10.34973/9txv-z787) instead of the journal paper DOI, causing the "Read Paper" button in the API docs to resolve to the wrong publication.

Change

• moabb/datasets/thielen2021.py: Replace the doi in Thielen2021.__init__ with the correct paper DOI:

# Before
doi="10.34973/9txv-z787",  # dataset repo DOI

# After
doi="10.1088/1741-2552/abecef",  # journal paper DOI

The dataset repository DOI is already correctly referenced via external_links and DocumentationMetadata.data_url elsewhere in the class.