Fix ShallowWrapper/simulate docs example. Was simulating on wrong node

- Adds `.find('a')` so the click is simulated on `<a />` rather than `<Foo />`

Referenced here: #88 (comment) by @ljharb

docs/api/ShallowWrapper/simulate.md

@@ -43,7 +43,7 @@ class Foo extends React.Component {
 const wrapper = shallow(<Foo />);
 
 expect(wrapper.find('.clicks-0').length).to.equal(1);
-wrapper.simulate('click');
+wrapper.find('a').simulate('click');
 expect(wrapper.find('.clicks-1').length).to.equal(1);
 ```