[Cashtab] Use XEC and not XECs

Summary: XEC is standard, even for plural, e.g. "1 XEC" and "100 XEC" are both grammatically correct

Test Plan: npm test, grep -r XECs src/ and no output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14329