Change name of parameter in documentation from sigret to sig

Change name of parameter in documentation from sigret to sig

The rest of the documentation talks about sig, not sigret

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18943)

(cherry picked from commit 2bd8190aace8109a06aff495a3e20c863ef48653)