Syntax error in CI docs for disabling/enabling builds globally

There's a typo in the enable_or_disable_ci document near the bottom discussion of globally enabling/disabling builds.

replace the dash with an underscore to match other gitlab_rails stanzas in /etc/gitlab/gitlab.rb for gitlab-ce omnibus installation.

-gitlab-rails['gitlab_default_projects_features_builds'] = false
+gitlab_rails['gitlab_default_projects_features_builds'] = false