Fix QueryConfig WriteDisposition field docs
Fix QueryConfig WriteDisposition field docs

The WriteDisposition field docs in the QueryConfig struct incorrectly
stated that the default TableWriteDisposition is WriteAppend when it is
actually WriteEmpty. I found this out the hard way :'(.

I confirmed that the default was WriteEmpty by looking at the rest docs:
https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs

Signed-off-by: Steven Erenst <stevenerenst@gmail.com>

Change-Id: I47040653ee4dcf8287a43bb396b48541130d4c05
Reviewed-on: https://code-review.googlesource.com/11691
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go