docs(email-response): fix typo

[michael-sumner]

Fixes typo in email-response.php

Proposed changes:

• Fix typo in email-response.php

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• N/A

[github-actions]

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Jetpack plugin:

• Next scheduled release: August 1, 2023.
• Scheduled code freeze: July 24, 2023.

[anomiex]

Thanks!