asciidoctor/extract-header-ids: make work with definition lists

These were failing because the plugin was copied from an outdated documentation
and definition lists don't have the .context method:
https://github.com/asciidoctor/asciidoctor/issues/3363

asciidoctor/extract-header-ids

@@ -8,17 +8,12 @@ require 'asciidoctor/extensions'
 class Main < Asciidoctor::Extensions::TreeProcessor
   def process document
     return unless document.blocks?
-    process_blocks document
-    nil
-  end
-
-  def process_blocks node
-    node.blocks.each_with_index do |block, i|
-      if block.context == :section
-        puts block.id
+    (document.find_by context: :section).each do |section|
+      if section.id
+        puts section.id
       end
-      process_blocks block if block.blocks?
     end
+    nil
   end
 end