[bitcoinabc.org] Update cashtab link to new URL

Summary:
T1910

Cashtab is moving from cashtabapp.com to cashtab.com

This diff updates the URL to the now-live cashtab.com

Test Plan:

1. Run locally
2. Navigate to ecash.html and verify that the Cashtab link goes to cashtab.com instead of cashtabapp.com

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10330