Update the search for the xml element we use for SCAP

The format of the xml file has changed so the existing search was
not finding any results.

This search works as of scap-security-guide version 0.1.46

https://bugzilla.redhat.com/show_bug.cgi?id=1769901

lib/linux_admin/scap.rb

@@ -83,7 +83,7 @@ def write_ds_xml(io, profile_xml)
     end
 
     def model_xml_element(doc)
-      doc.css("//nist_list|model", "nist_list" => "http://checklists.nist.gov/xccdf/1.2").detect { |model| model.namespace.prefix.nil? }
+      doc.xpath("//ns10:model").first
     end
   end
 end