docs: fix types argument passed to Domain in README

The value of type dict passed to the Domain constructor in the README is set to None, denoting the fact that the type has no parent type.

README.md

@@ -104,7 +104,7 @@ a1 = Action(
 requirements = [Requirements.STRIPS, Requirements.TYPING]
 domain = Domain("my_domain",
        requirements=requirements,
-       types={"type_1": []},
+       types={"type_1": None},
        constants=[a, b, c],
        predicates=[p1, p2],
        actions=[a1])