Category index page is missing slash

[ssbarnea]

I am using htmltest to validate the build output and I found what it appears to be a bug in theme:

htmltest started at 01:37:11 on public
========================================================================
running in concurrent mode, this is experimental
categories/romanian/index.html
  target is a directory, href lacks trailing slash --- categories/romanian/index.html --> /categories
categories/tools/index.html
  target is a directory, href lacks trailing slash --- categories/tools/index.html --> /categories
categories/devops/index.html
  target is a directory, href lacks trailing slash --- categories/devops/index.html --> /categories
categories/i18n/index.html
  target is a directory, href lacks trailing slash --- categories/i18n/index.html --> /categories
categories/molecule/index.html
  target is a directory, href lacks trailing slash --- categories/molecule/index.html --> /categories
categories/ansible/index.html
  target is a directory, href lacks trailing slash --- categories/ansible/index.html --> /categories
categories/python/index.html
  target is a directory, href lacks trailing slash --- categories/python/index.html --> /categories
categories/usability/index.html
  target is a directory, href lacks trailing slash --- categories/usability/index.html --> /categories
========================================================================

I may make sense to run htmltest on example site as part of the CI testing to assure that that theme output is "more" valid ;)