Fix accumulated index comments in felem_inv for p521

Fix accumulated index comments in felem_inv for p521

Comments in felem_inv refer to the logarithm with respect to in of the
element that has just been computed. The last two such annotations are
incorrect. By Fermat's last theorem, we hope to compute in^(p-2) in
GF(2^521-1), as such we expect the final index we reach to be 2^521-3.

CLA: Trivial

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19765)