Fix astuple in dataclasses documentation (GH-9631)

(cherry picked from commit 508d820512)

Co-authored-by: 방성범 (Bang Seongbeom) <bangseongbeom@hotmail.com>

Doc/library/dataclasses.rst

@@ -309,7 +309,7 @@ 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