The sub-index calculation was incorrect when demonstrating how to map a validator in the sync committee to a particular bit in the `aggregation_bits` in the sync committee aggregation scheme. The correct index is given in this commit.
The field name `aggregation_bits` was changed from `aggregate_bits` to reflect the same usage under the attestation scheme. This PR cleans up left-over references to the old field name in the section header and TOC.