Fix various mistakes in ec_GFp_nistp_recode_scalar_bits comment.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9050)

(cherry picked from commit 8be6a4ed02297f380bbea269f2e1c08a592444bc)