Fix styleguide on markdown.rb

Author: rafaelfranca

guides/rails_guides/markdown.rb

@@ -128,7 +128,7 @@ def generate_structure
               node.inner_html = "<a class='anchorlink' href='##{node[:id]}' data-turbo='false'>#{node.inner_html}</a>"
             end
 
-            tables = doc.css('table')
+            tables = doc.css("table")
             tables.wrap("<div class='table-wrapper'>")
           end
           @body = @epub ? document.to_xhtml : document.to_html