Fix grammatical errors

[Marcofann]

This pull request addresses grammatical issues in comments across the codebase, specifically related to the correct usage of articles ("a" vs. "an"). The affected files are:

• ec/src/models/twisted_edwards/affine.rs
• poly/src/domain/general.rs
• poly/src/domain/utils.rs

Changes made:

1. Replaced "an y-coordinate" with "a y-coordinate" in affine.rs.
2. Replaced "a FFT-friendly finite field" with "an FFT-friendly finite field" in general.rs.
3. Replaced "evaluations of a over the coset" with "evaluations of an over the coset" in utils.rs.

These changes improve the clarity and grammatical correctness of the code documentation.