[ecash-wallet] Patch README to use correct import syntax and fromMnemonic constructor
Summary: We no longer export Wallet default, so we need to import in brackets. Use the new fromMnemonic constructor in the README which will probably be the more common instantiation.
Test Plan: proofread
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D18391