Update workaround for "Outside the Scope of Strong Parameters"

The previous example of how to permit a hash of unknown keys used .tap, but had the side effect of logging an "Unpermitted parameters" message despite being a successful workaround. The proposed workaround is ever so slightly better, imo, because it won't result in an "Unpermitted parameters" message being logged.

guides/source/action_controller_overview.md

@@ -328,9 +328,7 @@ the job done:
 
 ```ruby
 def product_params
-  params.require(:product).permit(:name).tap do |whitelisted|
-    whitelisted[:data] = params[:product][:data]
-  end
+  params.require(:product).permit(:name, { data: params[:product][:data].keys })
 end
 ```