Tracked this down in the repo in the doc directory.

There does not appear to be a spec.txt, nor the image
files it needs in the current head.

I changed the spec.html references to design.html and changed the
spec.html reference in installation.txt to point to design.html as
well.

design.txt's reference to spec.html still exists. I was able to find
what I think is spec.html from google cache. Details in the check-in
message below. However I am not sure how to update the web site to
get my changed .txt files. I am not sure how to (or if I should)
integrate the spec.html I downloaded into the roundup web site.

I pushed my changes to ssh://hg.code.sf.net/p/roundup/code

check-in note:

changeset:   4948:4a3a35192ad1
tag:         tip
parent:      4945:55aef7ab35a8
user:        John Rouillard <rouilj at ...>
date:        Sat Dec 06 23:18:40 2014 -0500
files:       doc/developers.txt doc/installation.txt doc/spec.html
description:
Fixing http://issues.roundup-tracker.org/issue2550859 which reports a
404 error accessing spec.html reference in the developer.html page.

Changed references to spec.html in developers.txt and installation.txt
to point to design.html since there is no spec.html.

I did track down a copy of what I think is supposed to be spec.html
from google cache of:

 http://zesty.ca/sc-roundup.html. It is a snapshot of the page as it
 appeared on Dec 4, 2014 22:56:45 GMT

obtained via:

http://webcache.googleusercontent.com/search?q=cache:Zyw8ZjphH2AJ:zesty.ca/sc-roundup.html

The copy of spec.html is missing images (www.lfw.org which hosted the
images seems to be gone).

I left the reference to spec.html in design.txt (aka design.html)
rather than having it point to itself. Also the text surrounding the
reference to spec.html discussses rationale and things that are
misssing from design.html.

I am not sure what can be done to retrieve the real spec.html with
images etc, or exactly how to handle the spec.html reference in
design.html.