[object Object] 'text' values for accordions on manual_section pages

What

Use only the 'text' attribute of the object used to render the heading of accordions on
manual sections.

Why

Currently, when the value arrives in Google Analytics (GA) the ga4['text'] value is
showing as '[object Object]' and not a simple string.

app/views/content_items/manual_section.html.erb

@@ -65,7 +65,7 @@
                       ga4: {
                         event_name: "select_content",
                         type: "accordion",
-                        text: item[:heading],
+                        text: item[:heading][:text],
                         index: index,
                         index_total: @content_item.main.length,
                       }