[FIX] Fixing template string on electrodes for eeg and ieeg.

[adam2392]

Summary

Closes: #649

Fixes the template strings in the modality-specific spec to match that of https://bids-specification.readthedocs.io/en/stable/99-appendices/04-entity-table.html#encephalography-eeg-ieeg-and-meg

[sappelhoff]

This is in accordance with the entity table: https://bids-specification.readthedocs.io/en/latest/99-appendices/04-entity-table.html#encephalography-eeg-ieeg-and-meg ... so this is truly a fix.

unfortunately, as discussed in this issue comment, the validator is not precise enough to validate this. So the validator will still accept files like: *_task-Nonsense_electrodes.tsv. There is a similar issue here: https://github.com/bids-standard/bids-validator/issues/848

Still +1 to merge, then adjust mne-bids (mne-tools/mne-bids#604), and then eventually improve the validator (better sooner than later)

[sappelhoff]

Thanks @adam2392