docs: update example usage (#68)

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Fixes #67 🦕

README.rst

@@ -105,7 +105,7 @@ Example Usage
    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}]
    })
 
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