Fix CoffeeScript example

`a[data-color]` is not used in example html.

guides/source/working_with_javascript_in_rails.md

@@ -111,7 +111,7 @@ paintIt = (element, backgroundColor, textColor) ->
     element.style.color = textColor
 
 $ ->
-  $("a[data-color]").click ->
+  $("a[data-background-color]").click ->
     backgroundColor = $(this).data("background-color")
     textColor = $(this).data("text-color")
     paintIt(this, backgroundColor, textColor)