Fix an error in the expression for Δ in Pedersen hashing, and add acknowledgement to Kobi Gurkan.
Fix an error in the expression for Δ in Pedersen hashing, and add acknowledgement to Kobi Gurkan.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips