[website] Fix cashtab-storybook alt tag

Summary:
Noticed this while reviewing D11623
Looks like it was accidentally copy-pasted from the cashtab-components image tag

Test Plan: Ran it locally, works fine

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11624