fix: svdw params prefer z = ±(1+n) to ±(i+n)

[Tabaie]

In accordance with version 14 of the standard:
https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-hash-to-curve-14#appendix-H.1

[Tabaie]

On a (slightly) different note,

# NOTE: if init_ctr=1 fails to find Z, try setting it to F.gen()

I find this rather vague as it is not specified how many trials constitute a failure.

[armfazh]

I find this rather vague as it is not specified how many trials constitute a failure.

These parameters are generated offline per-field & per-curve.
So implementations shouldn't calculate these values online, (this library does calculate them, but only for compatibility reasons as this is the reference implementation).

[Tabaie]

Makes sense, thanks!