[Cashtab] Patch typo in ScanQRCode

Summary:
Related to T2616. In ScanQRCode at line 72, the variable isToken must return true in order to generate the appropriate google analytics event label. As there is no key value pair in the currency object for tokenPrefix, isToken has been returning false, thus not generating the eventLabel. The typo has been corrected to currency.tokenPrefixes, which allows the code that follows to function correctly.

generate a QR code with an eToken address

Test Plan:
cd web/cashtab && npm start

at line 73, add console.log(isToken)

navigate to send token screen
click the QR scanner button on the address input
scan the eToken address QR code generated in earlier step
observe that isToken returns true on scan

changes planned while testing from the GA dashboard

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11881