docs: change example to be like GUIDELINES.md

[betinacosta]

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Description

There was a mismatch between GUIDELINES.md where the recommendation was to name models as _models.py but in
CONTRIBUTING.md was only models.py. As we are using _models.py in most of our modules (exept DMS, to fixed later), I changed the CONTIBUTING.md.

Related Issue

Closes #72

(#72)

Type of Change

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• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

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Checklist

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• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

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• What breaks
• Migration path for users
• Alternative approaches considered

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