Fix rails guides generation

Was getting `formats=': Invalid formats: "html"

guides/rails_guides/generator.rb

@@ -164,7 +164,7 @@ def generate_guide(guide, output_file)
 
           # Generate the special pages like the home.
           # Passing a template handler in the template name is deprecated. So pass the file name without the extension.
-          result = view.render(layout: layout, formats: [$1], file: $`)
+          result = view.render(layout: layout, formats: [$1.to_sym], file: $`)
         else
           body = File.read("#{@source_dir}/#{guide}")
           result = RailsGuides::Markdown.new(