fix: confusing lang message `invalidEmail`

[kenjis]

Description See https://github.com/codeigniter4/shield/pull/1165#discussion_r1719317505

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [x] Conforms to style guide