Also update 'start_page' references to work with Antora v3

antora.yml

@@ -9,7 +9,7 @@ title: Fedora Magazine # <---- PLEASE MODIFY
 version: master
 
 # We encourage you to name the index page as "index.adoc". If you absolutely have to use a different name, please reflect it here. You can ignore this field otherwise. 
-start_page: ROOT:index
+# start_page: ROOT:index.adoc
 
 # This lists all the menu definitions of your component.
 nav:

site.yml

@@ -1,6 +1,6 @@
 site: 
   title: Fedora Magazine
-  start_page: fedora-magazine::index
+  start_page: fedora-magazine::index.adoc
 content:
   sources:
    - url: .