[Cashtab] Remove broken link from README

Summary: Remove broken link. This link was missed when screenshots folder was deleted.

Test Plan:
Confirm this change only impacts README.md
Confirm the broken link is removed and no typos are introduced

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12727