DOCS-10632: upserted response includes _id not nec. ObjectId

Author: schmalliso

source/reference/command/findAndModify.txt

@@ -285,7 +285,7 @@ and returns a document with the following fields:
 
 - The ``lastErrorObject`` field that contains the details of the
   command, including the field ``upserted`` that contains the
-  ``ObjectId`` of the newly inserted document, and
+  ``_id`` value of the newly inserted document, and
 
 - The ``value`` field containing ``null``.