[ecash-wallet] Correct main entry to refer to built typescript

Summary:
there is no index.js, after build there is a dist/index.js; patch

This was not noticed in the cashtab-faucet implementation as the local install works on the unbuilt files

Test Plan: proofread

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18390