Do not gsub non ASCII characters in header anchor.

guides/rails_guides/markdown.rb

@@ -47,8 +47,7 @@ def dom_id(nodes)
       end
 
       def dom_id_text(text)
-        text.downcase.gsub(/\?/, '-questionmark').gsub(/!/, '-bang').gsub(/[^a-z0-9]+/, ' ')
-          .strip.gsub(/\s+/, '-')
+        text.downcase.gsub(/\?/, '-questionmark').gsub(/!/, '-bang').gsub(/\s+/, '-')
       end
 
       def engine