Fix documentation error in x509 app certopt flag

According to the x509 man page in the section discussing -certopt it says
that the ca_default option is the same as that used by the ca utility and
(amongst other things) has the effect of suppressing printing of the
signature - but in fact it doesn't. This error seems to have been present
since the documentation was written back in 2001. It never had this effect.

The default config file sets the certopt value to ca_default. The ca utility
takes that and THEN adds additional options to suppress printing of the
signature. So the ca utility DOES suppress printing of the signature - but
it is not as a result of using the ca_default option.

GitHub Issue #247

Reviewed-by: Rich Salz <rsalz@openssl.org>