docs: fixed 'annotate an image' (#330)

Author: galz10

docs/index.rst

@@ -17,7 +17,7 @@ You can call the :meth:`annotate_image` method directly:
  >>> client = vision.ImageAnnotatorClient()
  >>> response = client.annotate_image({
  ... 'image': {'source': {'image_uri': 'gs://my-test-bucket/image.jpg'}},
- ... 'features': [{'type': vision.enums.Feature.Type.FACE_DETECTION}],
+ ... 'features': [{'type_': vision.Feature.Type.FACE_DETECTION}],
  ... })
  >>> len(response.annotations)
  2