Fixed decorator sample (thanks tixz)

* TextDecorator was written in JavaScript-style function assignment,
  making the @processors variable drop out of scope

* Rewrote it as class TextDecorator and included a constructor to set
  the var

Author: dbrady

chapters/design_patterns/decorator.md

@@ -26,7 +26,9 @@ miniMarkdown = (line) ->
 stripComments = (line) ->
     line.replace /\s*\/\/.*$/, '' # Removes one-line, double-slash C-style comments
 
-TextProcessor = (@processors) ->
+class TextProcessor
+    constructor: (@processors) ->
+
     reducer: (existing, processor) ->
         if processor
             processor(existing or '')