Small fixup of util/process_docs.pl

Apparently, pod2html doesn't add ".html" at the end of links, making
them useless, so we need to fix that

With thanks for the report to Michel <michel.sales@free.fr>

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1897)