Fix off-by-one in expected approved block-descriptions

[rphmeier]

We're getting many mismatch alerts from the relay-chain selection logic.

For example: 'Mismatch of anticipated block descriptions and block number difference. present_block_descriptions=59, target_number=8869933, subchain_number=8869991"

As 59+8869933=8869992, there's an off-by-one difference in what approval voting is producing and what the dispute coordinator is expecting. Or possibly, the sanity check has an off-by-one issue. This needs verification against intended behavior and a fix either to approval-voting or the sanity-check.