doc: Get rid of unrelated reference to DSA_new()

CLA: trivial

Signed-off-by: Jakub Jelen <jjelen@redhat.com>

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8786)