Update RAND_METHOD definition in man page

Update RAND_METHOD definition in man page

The `add` and `seed` callbacks were changed to return `int` instead of
`void` in b6dcdbfc94c482f6c15ba725754fc9e827e41851 (first included in
tag OpenSSL_1_1_0-pre1).

The `add` callback was changed to take a `double` instead of an `int`
in 853f757ecea74a271a7c5cdee3f3b5fe0d3ae863.

CLA: trivial
Fixes: #10199

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11486)