Add missing HTML tag in www_body in s_server.c

In the generated HTML document, the `<pre>` tag is not closed.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4088)
(cherry picked from commit 1a9f5cf0)