[docs] Fix type on renderIntoDocument

Fixes #3581.

docs/docs/10.4-test-utils.md

@@ -30,7 +30,7 @@ React.addons.TestUtils.Simulate.keyDown(node, {key: "Enter"});
 ### renderIntoDocument
 
 ```javascript
-ReactComponent renderIntoDocument(ReactComponent instance)
+ReactComponent renderIntoDocument(ReactElement instance)
 ```
 
 Render a component into a detached DOM node in the document. **This function requires a DOM.**