fft/domain: fix stale comment (FinerGenerator/GeneratorSqrt)

[radik878]

The FFT domain template referenced non-existent identifiers “FinerGenerator” and “GeneratorSqrt”.
These names do not exist anywhere in the codebase and originated from outdated comments in the
template, causing the incorrect text to be propagated to all generated fft/domain.go files.

This change updates the template to accurately describe that we compute a 2^k-th root of unity
and store it in Generator, and other values are derived from it (e.g., GeneratorInv).

Rationale:

• Eliminates misleading documentation that can confuse readers and contributors.
• Ensures future code generation produces correct comments across all curves/fields.
• No functional changes, code behavior and APIs remain unaffected.