Swap fileserver wiki link for fileserver docs

Many pages point to a wiki page for more information concerning fileserver
configuration:

  http://projects.puppetlabs.com/projects/puppet/wiki/File_Serving_Configuration

This page has been decomissioned and now points to an official puppet-docs
guide:

  http://docs.puppetlabs.com/guides/file_serving.html

This commit swaps the wiki links out for a direct link to the official docs.