Update THESHOLD name in jacobi.c

[Niels Möller]

nisse at lysator.liu.se (Niels Möller) writes:

> Algorithm description was most recently posted here:
> https://gmplib.org/list-archives/gmp-bugs/2017-August/004190.html

Now published on arxiv, with only minor updates.
https://arxiv.org/abs/1907.07795

Regards,
/Niels

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