fix faucet link

[mashharuki]

Overview

Fixed the outdated Testnet Faucet link in the documentation. The previous link was incorrect/broken, preventing users from acquiring test tokens. Updating this ensures a smoother onboarding experience for developers testing on the network.

Submission Checklist

• Useful pull request description
• Tests are provided (if possible)
• Key commits have useful messages
• All check jobs of the CI have succeeded
• Self-reviewed the diff
• Reviewer requested
• Update README file (if relevant)
• Update documentation (if relevant)
• No new TODOs introduced

Closes #10

Current faucet url: https://faucet.preprod.midnight.network/

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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