Update README Example Usage for v2

[dylancaponi]

Environment details

• OS type and version: macOS Mojave 10.14.6
• Python version: Python 3.8.5
• pip version: pip 20.2.3
• google-cloud-vision version: 2.0.0

Steps to reproduce

1. Run the example usage code: https://github.com/googleapis/python-vision#example-usage
2. Get error 'google.cloud.vision' has no attribute 'enums'

Guessing this has to do with: https://googleapis.dev/python/vision/latest/UPGRADING.html

Code example

from google.cloud import vision

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}],
})

Stack trace

Traceback (most recent call last):
  File "example.py", line 6, in <module>
    'features': [{'type': vision.enums.Feature.Type.FACE_DETECTION}],
AttributeError: module 'google.cloud.vision' has no attribute 'enums'

[munkhuushmgl]

@dylancaponi It is due to the breaking changes https://googleapis.dev/python/vision/latest/UPGRADING.html#enums-and-types

enums and types are already moved, please refactor on your end accordingly.