lib/rdoc/markup/to_html.rb (accept_heading): Fix page-top link.

99f7210 introduced jump-to-top link with ID `top`. 5f15520 introduced page-top link without corresponding target element. Should probably reuse existing ID.
Merge branch 'fix-page-top-link' of https://github.com/blackwinter/rdoc into blackwinter-fix-page-top-link
Merge branch 'blackwinter-fix-page-top-link'
Add #329 to History