fix: Update FormNewsletterSignup.vue - fix regex

[jamesray]

Checklist

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• [ ] Link to the issue if this PR is issue-specific.
• [ ] Create/update the corresponding story if this includes a UI component.
• [ ] Create/update documentation. If not included, tell us why.
• [ ] List the environments / browsers in which you tested your changes.
• [ ] Tests, linting, or other required checks are passing.
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Description

Instructions to test

Tested in the following environments/browsers:

Operating System

• [ ] macOS
• [ ] iOS
• [ ] iPadOS
• [ ] Windows

Browser

• [ ] Chrome
• [ ] Firefox ESR
• [ ] Firefox
• [ ] Safari
• [ ] Edge