[ecashaddrjs] Patch bug in README link to correct display at npm listing

Summary:
T3119

Correct README link so that ecashaddrjs npm page displays the README

Test Plan: Confirm that updated link goes to github page of modules/ecashaddrjs

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13698