Fix grammatical typos and improve clarity in comments across multiple files

[Pricstas]

This pull request fixes grammatical issues related to the use of the articles "a" and "an" in code comments. The following files were updated:

• ecc/bls12-377/fr/sis/sis.go: Corrected "an slice" to "a slice."
• ecc/bls12-377/internal/fptower/e2.go: Fixed article usage.
• ecc/bls12-377/internal/fptower/e6.go: Corrected article usage.
• utils/supsub.go: Corrected "a error" to "an error."

These changes improve clarity and ensure proper grammatical structure in the code comments.

Fixes # (issue)

Type of change:

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

• Manual review of code comments for grammatical correctness.

Checklist:

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works (if applicable).
• I did not modify files generated from templates.
• golangci-lint does not output errors locally.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.