Fixing the broken readme links [skip ci]

The README Links for activerecord & actionapack are broken on page http://api.rubyonrails.org/, throwing 404.  Since Links starting with link: refer to local files and hence they are forming these links http://api.rubyonrails.org/activerecord/README.rdoc and http://api.rubyonrails.org/actionpack/README.rdoc.  Providing the absolute github path.