Fix broken credentials_json example (#6)

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When authenticating with `credentials_json`, if you do not specify the
`token_format` the build breaks. I added the fix to the example.


![image](https://user-images.githubusercontent.com/1341777/190628523-c724f492-45e3-4165-a619-8ce9c5c631b8.png)

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Signed-off-by: Seth Vargo <[email protected]>
Co-authored-by: Seth Vargo <[email protected]>

Author: davidyun

workflows/deploy-cloudrun/cloudrun-docker.yml

@@ -85,6 +85,7 @@ jobs:
       #   uses: 'google-github-actions/auth@v0'
       #   with:
       #     credentials_json: '${{ secrets.GCP_CREDENTIALS }}'
+      #     token_format: 'access_token'
 
       # BEGIN - Docker auth and build (NOTE: If you already have a container image, these Docker steps can be omitted)