Fix doc to remove const arg in GEN_SESSION_CB

ae3947de

 changed the callback arg not to have a const parameter.

CLA: trivial

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

(cherry picked from commit 7588660a534a9f154e1e25aa763964d507af8aab)