Removed the "namespaced-attributes" ref from the DataDog/chef-datadog cookbook spec in Cheffile

The `bundle exec librarian-chef install` command was failing with:

    /.../1.9.3-p0/lib/ruby/gems/1.9.1/gems/librarian-0.0.26/lib/librarian/action/persist_resolution_mixin.rb:11:in `persist_resolution': undefined method `correct?' for nil:NilClass (NoMethodError)
      from /Users/karmi/.rbenv/versions/1.9.3-p0/lib/ruby/gems/1.9.1/gems/librarian-0.0.26/lib/librarian/action/resolve.rb:27:in `run'
      ....

Also:

    bundle exec librarian-chef update
    /.../1.9.3-p0/lib/ruby/gems/1.9.1/gems/librarian-0.0.26/lib/librarian/source/git/repository.rb:201:in `block in run_command_internal': fatal: ambiguous argument 'namespaced-attributes': unknown revision or path not in the working tree. (StandardError)
    Use '--' to separate paths from revisions
      from /Users/karmi/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/open3.rb:208:in `popen_run'
      ...