docs: Change tutorials to use identity column instead of serial/sequence

Resolves https://github.com/PostgREST/postgrest-docs/issues/311

docs/tutorials/tut0.rst

@@ -149,7 +149,7 @@ Our API will have one endpoint, :code:`/todos`, which will come from a table.
 .. code-block:: postgres
 
   create table api.todos (
-    id serial primary key,
+    id int primary key generated by default as identity,
     done boolean not null default false,
     task text not null,
     due timestamptz