Merge pull request facebook#5767 from yhagio/patch-2

Update 10.4-test-utils.md

Author: zpao

docs/docs/10.4-test-utils.md

@@ -232,6 +232,7 @@ After `render` has been called, returns shallowly rendered output. You can then
 Then you can assert:
 
 ```javascript
+var renderer = ReactTestUtils.createRenderer();
 result = renderer.getRenderOutput();
 expect(result.type).toBe('div');
 expect(result.props.children).toEqual([