Replace the 'SSL' broken link with SSL_CTX_set_security_level which seems not...

Replace the 'SSL' broken link with SSL_CTX_set_security_level which seems not being referenced from elsewhere
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1898)

doc/man7/ssl.pod

@@ -767,6 +767,7 @@ L<SSL_CTX_set_msg_callback(3)>,
 L<SSL_CTX_set_options(3)>,
 L<SSL_CTX_set_quiet_shutdown(3)>,
 L<SSL_CTX_set_read_ahead(3)>,
+L<SSL_CTX_set_security_level(3)>,
 L<SSL_CTX_set_session_cache_mode(3)>,
 L<SSL_CTX_set_session_id_context(3)>,
 L<SSL_CTX_set_ssl_version(3)>,