Fix createNodeMock doc to avoid Invariant Violation (#8989)

_posts/2016-11-16-react-v15.4.0.md

@@ -75,6 +75,7 @@ import renderer from 'react-test-renderer';
 function createNodeMock(element) {
   if (element.type === 'input') {
     return {
+      nodeType: 1,
       focus() {},
     };
   }