Adjust the default value of the private key size

...in the man page to reflect the actual default (2048 instead of 512)

CLA: trivial
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2754)

(cherry picked from commit 013bc448)