big query documentation link broken

[davidraleigh]

Steps to reproduce:
Click on link in README.md for bigquery at this line:
https://github.com/GoogleCloudPlatform/google-cloud-python/blame/master/bigquery/README.rst#L12

That line points to this link which is broken:
https://googlecloudplatform.github.io/google-cloud-python/stable/bigquery-usage.html

[dhermes]

Fixed by #3611