[non-substantive] Clarify offsets serialization

[ColinSchwarz]

https://github.com/ethereum/eth2.0-specs/pull/2043

[dapplion]

@wemeetagain Does this issue require any changes in our case? It just adds a uint32 casting

variable_offsets = [serialize(uint32(sum(fixed_lengths + variable_lengths[:i]))) for i in range(len(value))]