Make RedCloth not convert double hyphens to emdashes. Closes #5292

As mentioned in http://redcloth.org/textile/writing-paragraph-text/#dashes
RedCloth converts double hyphens to emdashes. This causes problems in
the guides where options like --database, --full are rendered incorrectly.

This commit fixes it by customizing the emdash method in the
RedCloth::Formatters::HTML module to just return '--'. See their FAQ
http://redcloth.org/faq in the section 'How do I customize RedCloth?'

guides/rails_guides/textile_extensions.rb

@@ -1,5 +1,11 @@
 require 'active_support/core_ext/object/inclusion'
 
+module RedCloth::Formatters::HTML
+  def emdash(opts)
+    "--"
+  end
+end
+
 module RailsGuides
   module TextileExtensions
     def notestuff(body)