[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