Update Bitcoin Cash to eCash in secp256k1 README

Summary: Left the specification reference as "Bitcoin Cash" for now until we have something else to refer to.

Test Plan: Read it, see that it makes sense.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: bobbyb815

Differential Revision: https://reviews.bitcoinabc.org/D9729