[Cashtab] corrected link in CreateTokenForm.js

Summary: Updated default token document URL as requested in task T1971.

Test Plan:
cd web/cashtab
npm start
navigate to the create token form
mint token that uses default URL, or click on URL for pre-existing token that uses the default URL
follow link

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10498