Fix ERA5 dataset link in training documentation

[alloutflo]

Summary

Update the README's retired ERA5 dataset URL to ECMWF's current "ECMWF Reanalysis v5 (ERA5)" page without changing the surrounding training-data guidance.

Closes #71

Validation

• confirmed the old URL is absent from README.md
• confirmed the replacement URL appears exactly once
• verified the replacement resolves to ECMWF's ERA5 dataset page through ECMWF's indexed page metadata
• git diff --check

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