chore(README.md): Grammar fix

[jokajak]

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
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• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:
Fix small grammar issue in README.md

How does this PR accomplish the above?:
Fix small grammar issue in README.md

What documentation changes (if any) are needed to support this PR?:
None

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• You must follow the template instructions. Failure to do so will result in your pull request being closed.
• Please respect that Pi-hole is developed by volunteers, who can only reply in their spare time.

[dschaper]

Thanks for the submission, please follow step one of the contributors guide.

https://github.com/pi-hole/pi-hole/blob/master/CONTRIBUTING.md#technical-requirements

[jokajak]

Looks like I missed the technical steps the first time through.

[pralor-bot]

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

https://discourse.pi-hole.net/t/pi-hole-core-web-v5-2-and-ftl-v5-3-released/40909/1