[DOCS] Remove reference for Asciidoctor migration

Author: jrodewig

docs/reference/migration/migrate_6_0/docs.asciidoc

@@ -6,7 +6,7 @@
 Document modification operations may no longer specify the `version_type` of
 `force` to override any previous version checks.
 
-==== <<upserts,Upserts>> no longer support versions
+==== Upserts no longer support versions
 
 Adding a `version` to an upsert request is no longer supported.