fix broken spec

spec/helpers/tree_helper_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe TreeHelper do
   describe '#markup?' do
-    %w(mdown md markdown textile rdoc org creole mediawiki rst asciidoc pod).each do |type|
+    %w(textile rdoc org creole mediawiki rst asciidoc pod).each do |type|
       it "returns true for #{type} files" do
         markup?("README.#{type}").should be_true
       end