/usr/lib/ruby/2.3.0/rdoc/markup/ is in libruby2.3 2.3.0-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/2.3.0/rdoc/markup/attr_changer.rb
/usr/lib/ruby/2.3.0/rdoc/markup/attr_span.rb
/usr/lib/ruby/2.3.0/rdoc/markup/attribute_manager.rb
/usr/lib/ruby/2.3.0/rdoc/markup/attributes.rb
/usr/lib/ruby/2.3.0/rdoc/markup/blank_line.rb
/usr/lib/ruby/2.3.0/rdoc/markup/block_quote.rb
/usr/lib/ruby/2.3.0/rdoc/markup/document.rb
/usr/lib/ruby/2.3.0/rdoc/markup/formatter.rb
/usr/lib/ruby/2.3.0/rdoc/markup/formatter_test_case.rb
/usr/lib/ruby/2.3.0/rdoc/markup/hard_break.rb
/usr/lib/ruby/2.3.0/rdoc/markup/heading.rb
/usr/lib/ruby/2.3.0/rdoc/markup/include.rb
/usr/lib/ruby/2.3.0/rdoc/markup/indented_paragraph.rb
/usr/lib/ruby/2.3.0/rdoc/markup/inline.rb
/usr/lib/ruby/2.3.0/rdoc/markup/list.rb
/usr/lib/ruby/2.3.0/rdoc/markup/list_item.rb
/usr/lib/ruby/2.3.0/rdoc/markup/paragraph.rb
/usr/lib/ruby/2.3.0/rdoc/markup/parser.rb
/usr/lib/ruby/2.3.0/rdoc/markup/pre_process.rb
/usr/lib/ruby/2.3.0/rdoc/markup/raw.rb
/usr/lib/ruby/2.3.0/rdoc/markup/rule.rb
/usr/lib/ruby/2.3.0/rdoc/markup/special.rb
/usr/lib/ruby/2.3.0/rdoc/markup/text_formatter_test_case.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_ansi.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_bs.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_html.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_html_crossref.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_html_snippet.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_joined_paragraph.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_label.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_markdown.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_rdoc.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_table_of_contents.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_test.rb
/usr/lib/ruby/2.3.0/rdoc/markup/to_tt_only.rb
/usr/lib/ruby/2.3.0/rdoc/markup/verbatim.rb