[ci skip] Remove faulty overrides in scrubber example.

Add initializer to CommentScrubber example replacing the method overrides with setting
tags and attributes.

Signed-off-by: Kasper Timm Hansen <kaspth@gmail.com>

README.md

@@ -99,17 +99,15 @@ You can also create custom scrubbers in your application if you want to.
 
 ```ruby
 class CommentScrubber < Rails::Html::PermitScrubber
-  def allowed_node?(node)
-    !%w(form script comment blockquote).include?(node.name)
+  def initialize
+    super
+    self.tags = %w( form script comment blockquote )
+    self.attributes = %w( style )
   end
 
   def skip_node?(node)
     node.text?
   end
-
-  def scrub_attribute?(name)
-    name == "style"
-  end
 end
 ```