doc/man1/x509.pod: fix typo

This looks like a copy&paste error from req.pod to x509.pod.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7995)

(cherry picked from commit 67ee899cb51d3e3d7b5f00b878f8f82a097b93f0)