Chore(optimizer): fix pushdown_predicates comment example

[georgesittas]

I updated the example so that the query we call pushdown_predicates on is equivalent to what we get by running the optimizer up to that step, not including it.

>>> import sqlglot
>>> from sqlglot.optimizer import optimize
>>> optimize("SELECT * FROM (SELECT * FROM x AS x) AS y WHERE y.a = 1", schema={"x": {"a": "int"}}).sql()
pushdown_predicates called with 'SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x) AS y WHERE y.a = 1'
'SELECT "x"."a" AS "a" FROM "x" AS "x" WHERE "x"."a" = 1'