Update --gen-x509 help text

[DL6ER]

What does this implement/fix?

Adds description for optional [domain]:

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Related issue or feature (if applicable): https://discourse.pi-hole.net/t/fix-pihole-ftl-help-output-for-gen-x509-option-with-domain-parameter/73622

Pull request in docs with documentation (if applicable): N/A

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By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• [x] The code change is tested and works locally.
• [x] I based my code and PRs against the repositories developmental branch.
• [x] I signed off all commits. Pi-hole enforces the DCO for all contributions
• [x] I signed all my commits. Pi-hole requires signatures to verify authorship
• [x] I have read the above and my PR is ready for review.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/fix-pihole-ftl-help-output-for-gen-x509-option-with-domain-parameter/73622/2

[rdwebdesign]

One test (armv7) failed.

[DL6ER]

I kicked the test - it is entirely unrelated, apparently, something was off with the testing container (Failed to receive data from NTP server 127.0.0.1: Connection refused).