util/process_docs.pl: handle multiple source directories correctly

The way this script handled multiple source directories wasn't quite
right, it ended up giving pod2html 'ARRAY(0xXXXXXXXXX)' as a source
directory.

This corrects the mistake.

Fixes #7742
Fixes #7939

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7911)