[cashtab-docs] correct url to match deployment to fix tiwtter card
[cashtab-docs] correct url to match deployment to fix tiwtter card

Summary: In prod, docs.cashtabapp.com is looking for the twitter card at `https://www.bitcoinabc.org/bcha/img/twitter-card.jpg`, instead of `https://docs.cashtabapp.com/img/twitter-card.jpg`. Correct the `url` parameter now that the deployment location for these docs is set.

Test Plan: `npm start`, observe app still loads. Will need to be deployed to observe if twitter card works on twitter.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D9204

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