Revise proposal 162: SHA256(x), not SHA256(SHA256(x))

Revise proposal 162: SHA256(x), not SHA256(SHA256(x))

The point of doing SHA256 twice is, generally, is to prevent message
extension attacks where an attacker who knows H(A) can calculate
H(A|B).  But for attaching a signature to a document, the attacker
already _knows_ A, so trying to keep them from calculating H(A|B) is
pointless.