Fix doc to remove const arg in GEN_SESSION_CB
Fix doc to remove const arg in GEN_SESSION_CB

ae3947de095 changed the callback arg not to have a const parameter.

CLA: trivial

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9104)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl