fix doc:readmes task

Was failing with "private method `mkdir' called for File:Class" when the
doc directory didn't exist.

sinatra-contrib/Rakefile

@@ -10,7 +10,7 @@ namespace :doc do
       next if file == 'lib/sinatra/contrib.rb'
       doc  = File.read(file)[/^module Sinatra\n(  #[^\n]*\n)*/m].scan(/^ *#(?!#) ?(.*)\n/).join("\n")
       file = "doc/#{file[4..-4].tr("/_", "-")}.rdoc"
-      File.mkdir "doc" unless File.directory? "doc"
+      Dir.mkdir "doc" unless File.directory? "doc"
       puts "writing #{file}"
       File.open(file, "w") { |f| f << doc }
     end
@@ -44,4 +44,4 @@ task 'sinatra-contrib.gemspec' do
   File.open('sinatra-contrib.gemspec', 'w') { |f| f << content }
 end
 
-task :gemspec => 'sinatra-contrib.gemspec'
+task :gemspec => 'sinatra-contrib.gemspec'