[4.2.x] Refs #34140 -- Applied rst code-block to non-Python examples.

Thanks to J.V. Zammit, Paolo Melchiorre, and Mariusz Felisiak for
reviews.

Backport of 534ac48297 from main.

docs/ref/contrib/postgres/expressions.txt

@@ -22,7 +22,9 @@ in the way that it does not act as an aggregate function and does not require
 an SQL ``GROUP BY`` clause to build the list of values.
 
 For example, if you want to annotate all related books to an author as JSON
-objects::
+objects:
+
+.. code-block:: pycon
 
     >>> from django.db.models import OuterRef
     >>> from django.db.models.functions import JSONObject