[e.cash] Change from Bitgo utxo-lib to ecash-lib on Build page

Summary: Now that it's implemented in Cashtab, we should recommend the new ecash-lib

Test Plan: Run it locally, check link

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16100