Fix bug in Horner's rule for Zeta
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Fix bug in Horner's rule for Zeta


This change used to produce an invalid result for the Hurwitz Zeta function.
It matches https://github.com/openxla/xla/blob/7a067a7b88d2ffb15b1dc5e3c06f701a15f0391d/xla/client/lib/math.cc#L1967-L1968 C++ implementation as well.

Example zeta(2, q=1):
jax32/zeta : 1.6449342966079712
jax64/zeta : 1.6449342726140739
scipy/zeta : 1.6449340668482266
pi^2 / 6   : 1.6449340668482264
After fixing:
jax64/zeta : 1.6449340668482264

Fixes #5838
Co-authored-by: pparuzel <paruzelp@google.com>
Co-authored-by: Adam-Banas <adambanas@google.com>
