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

doc/man1/genrsa.pod

@@ -79,7 +79,7 @@ for all available algorithms.
 =item B<numbits>
 
 the size of the private key to generate in bits. This must be the last option
-specified. The default is 512.
+specified. The default is 2048.
 
 =back