[4.0.x] Updated example of YAML serialization format in docs.

Backport of feeb0685c6 from main

docs/topics/serialization.txt

@@ -330,9 +330,10 @@ YAML serialization looks quite similar to JSON. The object list is serialized
 as a sequence mappings with the keys "pk", "model" and "fields". Each field is
 again a mapping with the key being name of the field and the value the value::
 
-    -   fields: {expire_date: !!timestamp '2013-01-16 08:16:59.844560+00:00'}
-        model: sessions.session
-        pk: 4b678b301dfd8a4e0dad910de3ae245b
+    - model: sessions.session
+      pk: 4b678b301dfd8a4e0dad910de3ae245b
+      fields:
+        expire_date: 2013-01-16 08:16:59.844560+00:00
 
 Referential fields are again represented by the PK or sequence of PKs.