Update THESHOLD name in jacobi.c

[Seth Troisi]

Per Niels' suggestion I'm taking a look at the Jacobi documentation.

I noticed that jacobi.c aliases GCD_DC_THRESHOLD then uses both names once.
https://gmplib.org/repo/gmp/file/tip/mpn/generic/jacobi.c#l177
https://gmplib.org/repo/gmp/file/tip/mpn/generic/jacobi.c#l196

I can't see any reason why they shouldn't match (and probably use the newer
name).

https://github.com/sethtroisi/libgmp/pull/4/files
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