[Cashab] Corrected proptype in QRCode

Summary: Related to task T2367. In QRCode.js, the proptype for isCashAddress shoud be PropTypes.bool.

Test Plan:
npm start
navigate go receive page
Observe that you do not see this error in dev console

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11289