[Cashtab] fix typo on new wallet screen

Summary: Fixed typo on new wallet screen, related to task T2031.

Test Plan:
cd web/cashtab
npm start
navigate to new wallet
ensure written copy in the Sign Up component matches what is in the task

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10617