FIX: Get only the correct collapse title in emails (#10228)

Previously, this would get every details block title from the entire post, for every details block.

plugins/discourse-details/plugin.rb

@@ -33,7 +33,7 @@ after_initialize do
 
   on(:reduce_cooked) do |fragment, post|
     fragment.css("details").each do |el|
-      text = fragment.css("summary").text
+      text = el.css("summary").text
       link = fragment.document.create_element("a")
       link["href"] = post.url if post
       link.content = I18n.t("details.excerpt_details")