docs: isSuccess is a boolean not a function (#1282)

Author: Juan gabriel ramirez cruz

docs/src/pages/docs/guides/testing.md

@@ -79,7 +79,7 @@ const expectation = nock('http://example.com')
 const { result, waitFor } = renderHook(() => useFetchData(), { wrapper });
 
 await waitFor(() => {
-  return result.current.isSuccess();
+  return result.current.isSuccess;
 });
 
 expect(result.current).toEqual({answer: 42});