Fix assertion error `set(previous_slot_duty_committies_index) == set(actual_committies_index)`

[nalepae]

Traceback (most recent call last):

  File "/usr/local/bin/eth-validator-watcher", line 8, in <module>
    sys.exit(app())

  File "/usr/local/lib/python3.10/site-packages/eth_validator_watcher/entrypoint.py", line 157, in handler
    ) = handle_missed_attestation_detection(

  File "/usr/local/lib/python3.10/site-packages/eth_validator_watcher/missed_attestations.py", line 79, in handle_missed_attestation_detection
    assert set(previous_slot_duty_committies_index) == set(actual_committies_index)

AssertionError

https://github.com/kilnfi/eth-validator-watcher/blob/main/eth_validator_watcher/missed_attestations.py#L81

The idea is to understand (and fix code accordingly) why sometimes, committies reported in slot n+1 do no correspond to committies duty of slot n