[Docs] invoke: Add missing backticks to end of codeblock

The example for invoke is missing a closing set of backticks for the example which is leading to it not rendering correctly on the docs site (https://airbnb.io/enzyme/docs/api/ShallowWrapper/invoke.html).

This commit updates the code example in . `invoke.md` to include the missing backticks.

docs/api/ShallowWrapper/invoke.md

@@ -38,3 +38,4 @@ const wrapper = shallow(<Foo />);
 wrapper.find('a').invoke('onClick')().then(() => {
   // expect()
 });
+```