Fix astuple in dataclasses documentation (GH-9631)

Doc/library/dataclasses.rst

@@ -308,7 +308,7 @@ Module-level decorators, classes, and functions
 
    Raises :exc:`TypeError` if ``instance`` is not a dataclass instance.
 
-.. function:: astuple(*, tuple_factory=tuple)
+.. function:: astuple(instance, *, tuple_factory=tuple)
 
    Converts the dataclass ``instance`` to a tuple (by using the
    factory function ``tuple_factory``).  Each dataclass is converted