examples/bigquery: update WriteDisposition field
examples/bigquery: update WriteDisposition field

Rename to match change in bigquery.Table.

Change-Id: Iad7574f6d81bf4c04287b2f9fe8d7aeb01615603
Reviewed-on: https://code-review.googlesource.com/8410
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go