TestUtils docs: isComponentOfType -> isDescriptorOfType

Fixes #1946

docs/09.4-test-utils.md

@@ -43,13 +43,13 @@ object mockComponent(function componentClass, string? tagName)
 
 Pass a mocked component module to this method to augment it with useful methods that allow it to be used as a dummy React component. Instead of rendering as usual, the component will become a simple `<div>` (or other tag if `mockTagName` is provided) containing any provided children.
 
-### isComponentOfType
+### isDescriptorOfType
 
 ```javascript
-boolean isComponentOfType(ReactComponent instance, function componentClass)
+boolean isDescriptorOfType(ReactDescriptor descriptor, function componentClass)
 ```
 
-Returns true if `instance` is an instance of a React `componentClass`.
+Returns true if `descriptor` is an instance of a React `componentClass`.
 
 ### isDOMComponent