Rename nested to nested group (#11658)

Author: noamr

css-view-transitions-2/Overview.bs

@@ -820,7 +820,7 @@ To resolve the [=used value=] of 'view-transition-name' for |element|:
 
 A 'view-transition-name' generated by <css>auto</css> is a [=tree-scoped name=].
 
-# Nested view-transitions # {#nested-view-transitions}
+# Nested view-transition groups # {#nested-view-transition-groups}
 
 ## Overview ## {#nested-overview}