multiprocessing.pool: fix ApplyResult.__init__ (#5318)

stdlib/multiprocessing/pool.pyi

@@ -9,6 +9,12 @@ _S = TypeVar("_S")
 _T = TypeVar("_T")
 
 class ApplyResult(Generic[_T]):
+    def __init__(
+        self,
+        pool: Pool,
+        callback: Optional[Callable[[_T], None]] = ...,
+        error_callback: Optional[Callable[[BaseException], None]] = ...,
+    ) -> None: ...
     def get(self, timeout: Optional[float] = ...) -> _T: ...
     def wait(self, timeout: Optional[float] = ...) -> None: ...
     def ready(self) -> bool: ...