Fix typographical errors and update broken link

[Marcofann]

This PR introduces the following changes to address minor issues in the codebase:

1. Fix Typographical Errors:

   • Updated grammar in constraints.rs to correct the use of the indefinite article "an" before words starting with vowel sounds.
   • Corrected a similar typo in error.rs for clarity and proper grammar usage.

2. Update Broken Link in README.md:

   • Replaced the outdated link to poly-commit/src/lib.rs with the correct link to poly-commit/poly-commit/src/lib.rs.

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Checklist

• Targeted PR against the correct branch (master).
• Linked to GitHub issue or provided explanation for changes (e.g., typo fixes and link corrections).
• Reviewed changes in Files changed section.
• Verified that all edits are consistent with the project's guidelines.

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Additional Notes

These updates are straightforward and improve the readability and usability of the project documentation and code. Please let me know if there are any additional changes or considerations before merging.