Fix broken link in the contributing documentation

Migrated issue

Reported by: rosk0

Related to !163 (merged)

Problem/Motivation

Broken links in:
* https://project.pages.drupalcode.org/gitlab_templates/help/help/ ==> contribute + documentation
* https://project.pages.drupalcode.org/gitlab_templates/help/contributing/ ==> templates version

None of the links are broken here https://project.pages.drupalcode.org/gitlab_templates/, but maybe it's worth having them start with "/" (which I assume will be the fix) to keep all internal links in the same format?

Proposed resolution

Fix the links.