doc/man3: remove a duplicate BIO_do_accept() call

The SSL server example in BIO_f_ssl.pod contains two copies of the
BIO_do_accept() call. Remove the second one.

Signed-off-by: Beat Bolli <dev@drbeat.li>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1956)