Fix path to clawlogo file.

When I introduced the file sidelogo.html, to make the logo link to index.html instead of to contents.html, I used a relative path, which caused the logo not to appear for pages that are not in the base directory of the docs.

In a recent commit, I tried to fix this by using a root-relative path, but I gave the wrong path.  This commit should fix it so the logo appears on all pages on clawpack.github.io, but unfortunately it does not fix it for viewing the pages locally.  I haven't figured out how to make both work.