Fixed typo in NodeSet spec.

spec/oga/xml/node_set_spec.rb

@@ -26,7 +26,7 @@ describe Oga::XML::NodeSet do
       node.node_set.should == set
     end
 
-    it 'onlies store unique nodes' do
+    it 'only store unique nodes' do
       n1  = Oga::XML::Element.new(:name => 'a')
       set = described_class.new([n1, n1])