[Tests] Fixes confusing test description on React 16.x

According to docs, calling `instance()` on a wrapped stateless component should return null

docs/api/ShallowWrapper/instance.md

@@ -30,7 +30,7 @@ class Stateful extends React.Component {
 ```
 #### React 16.x
 ```jsx
-test('shallow wrapper instance should not be null', () => {
+test('shallow wrapper instance should be null', () => {
   const wrapper = shallow(<Stateless />);
   const instance = wrapper.instance();