Refs #30511 -- Updated docs about auto-incrementing primary keys on PostgreSQL.

Follow up to 2eea361eff.

docs/topics/db/models.txt

@@ -39,7 +39,7 @@ The above ``Person`` model would create a database table like this:
 .. code-block:: sql
 
     CREATE TABLE myapp_person (
-        "id" serial NOT NULL PRIMARY KEY,
+        "id" bigint NOT NULL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
         "first_name" varchar(30) NOT NULL,
         "last_name" varchar(30) NOT NULL
     );