[e.cash] change MUSD to TUSDT on use-ecash page

Summary: The dollar.mp website has been updated to TUSDT, which is the token that is actually being used. MUSD was just a ceremonial token.

Test Plan: visit e.cash/use-ecash and check for typos or errors

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18239