Fix guide anchor generation logic.

This could use in any translation because it's not limited to `[^a-z0-9]`.

Verify after this change, all guides still have same anchors.

[Robin Dupret & Juanito Fatas]

guides/rails_guides/markdown.rb

@@ -47,7 +47,12 @@ module RailsGuides
       end
 
       def dom_id_text(text)
-        text.downcase.gsub(/\?/, '-questionmark').gsub(/!/, '-bang').gsub(/[^a-z0-9]+/, ' ').strip.gsub(/\s+/, '-')
+        escaped_chars = Regexp.escape('\\/`*_{}[]()#+-.!:,;|&<>^~=\'"')
+
+        text.downcase.gsub(/\?/, '-questionmark')
+                     .gsub(/!/, '-bang')
+                     .gsub(/[#{escaped_chars}]+/, ' ').strip
+                     .gsub(/\s+/, '-')
       end
 
       def engine