doc/man1/x509.pod: corrected "S/MIME signing" requirements

When the "certificate purpose" is checked and KeyUsage extension is present,
either 'digitalSignature' or 'nonRepudiation' is accepted.

Manual page corrected to reflect the above.

Signed-off-by: Konstantin Shemyak <konstantin@shemyak.com>

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5727)