docs: fix broken links

[osrm]

Motivation

The original links to the MetaMask private key export guide and Etherscan API key page were outdated or broken.

Change Summary

Replaced two outdated URLs with their current valid equivalents in the Goerli deployment section.

Merge Checklist

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• The PR title adheres to the conventional commits standard
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Additional Context

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PR-Codex overview

This PR updates the CONTRIBUTING.md file by correcting and clarifying the instructions for obtaining the GOERLI_PRIVATE_KEY and ETHERSCAN_KEY. The links provided for exporting keys have also been updated.

Detailed summary

• Updated the link for GOERLI_PRIVATE_KEY export instructions to a more relevant source.
• Corrected the link for ETHERSCAN_KEY export instructions by removing a period at the end.

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