Fixes incorrect destructuring

[hynky1999]

I have been playing with your library and I was unable to use any concepts because of following errors:

  File "/Users/hynekkdylicek/.pyenv/versions/dev/lib/python3.11/site-packages/lilac/concepts/concept.py", line 365, in _compute_embeddings
    for id, (embedding,) in zip(missing_ids, missing_embeddings):
            ^^^^^^^^^^^
ValueError: too many values to unpack (expected 1)

The problem is that the missing_embeddings can contain multiple embeddings (which I assume happens at long documents):

[{'vector': array([ 6.82974933e-...e=float32), 'span': (...)}, {'vector': array([ 4.36740592e-...e=float32), 'span': (...)}]

This PR fixes this by correctly desctructuring the touple

[dsmilkov]

Awesome catch, thank you!