Fix layouts and rendering guide example using flash.now instead

Thanks @lintzchiang. [ci skip]

guides/source/layouts_and_rendering.md

@@ -629,7 +629,7 @@ def show
   @book = Book.find_by_id(params[:id])
   if @book.nil?
     @books = Book.all
-    flash[:alert] = "Your book was not found"
+    flash.now[:alert] = "Your book was not found"
     render "index"
   end
 end