fix : Update local setup doc

[etiennejouan]

No description provided.

[greptile-apps]

PR Summary

Updated PostgreSQL credentials in local setup documentation to match default installation values, ensuring consistency across the development environment setup process.

• Changed database credentials in packages/twenty-website/src/content/developers/local-setup.mdx from user/password 'twenty' to 'postgres'/'postgres' to match .env.example and Docker configuration
• Aligned documentation with actual default PostgreSQL installation credentials for smoother first-time setup
• Ensured consistency with existing database connection strings in .env.example and .env.test

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