[5.1.x] Tweaked docs to avoid reformatting given new black version.

Backport of fd3cfd80be from main.

docs/ref/contrib/postgres/aggregates.txt

@@ -49,11 +49,10 @@ General-purpose aggregation functions
 
         Examples::
 
-            "some_field"
-            "-some_field"
             from django.db.models import F
 
-            F("some_field").desc()
+            ArrayAgg("a_field", order_by="-some_field")
+            ArrayAgg("a_field", order_by=F("some_field").desc())
 
     .. versionchanged:: 5.0