Merge branch 'fix-danger-docs' into 'master'

Fix the documentation Dangerfile

See merge request gitlab-org/gitlab-ce!25345

danger/documentation/Dangerfile

 # frozen_string_literal: true
 
-docs_paths_to_review = helper.changes_by_category[:documentation]
+docs_paths_to_review = helper.changes_by_category[:docs]
 
 unless docs_paths_to_review.empty?
   message 'This merge request adds or changes files that require a ' \