fix confusingly named function parameter 'scriptSig'

Summary:
scriptSig normally refers to a script, in this case it
was referring to a serialized signature.

Test Plan: trivial change

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2349