Small fixups of util/process_docs.pl

- the pod path hadn't been changed with the directory layout change
- 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/1896)