[Cashtab] Replace remaining instances of CashTab with Cashtab

Summary: Some instances of 'CashTab' vs the standardized name 'Cashtab' (per Chrome store, logos) discovered while writing docs. Replace with Cashtab.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9136