Fix doc nits in X509_check_private_key.pod

remove the tailing dot

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3614)