fix link to contributing guides

[luizirber]

Contributing guides had an extra /container-tree in the links:
https://singularityhub.github.io/container-tree/container-tree/contributing/docs/
instead of
https://singularityhub.github.io/container-tree/contributing/docs/

Removed {{ site.baseurl }} from links, and now they should be working.

(part of the JOSS review)

[vsoch]

hey @luizirber ! I just tested locally, and I think what we want to do is keep site.baseurl, and remove site.url. For example, here is the current PR:

and then when I change site.url to site.baseurl, all is well!

Are you able to reproduce this locally? This is a good fix as long as we get it right. :)

[luizirber]

My bad, I didn't have jekyll locally (or even a sane ruby environment... working on fixing that). Seems to be working now!

[vsoch]

No worries - we’re a team! :)