Trivial: Fix spelling in zapwallettxes test description

Summary:
Backport of core PR12212
https://github.com/bitcoin/bitcoin/pull/12212/files

Test Plan: Read the file doc.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3455