Add example label to activesupport/configurable

activesupport/lib/active_support/configurable.rb

@@ -33,6 +33,8 @@ def #{name}=(value); config.#{name} = value; end
 
     # Reads and writes attributes from a configuration <tt>OrderedHash</tt>.
     # 
+    # Example:
+    # 
     #   require 'active_support/configurable'      
     #  
     #   class User