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 <[email protected]>

Author: jba

examples/bigquery/load/main.go

@@ -66,7 +66,7 @@ func main() {
 
 	// Load data from Google Cloud Storage into a BigQuery table.
 	loader := table.LoaderFrom(gcs)
-	loader.TableWriteDisposition = bigquery.WriteTruncate
+	loader.WriteDisposition = bigquery.WriteTruncate
 	job, err := loader.Run(ctx)
 
 	if err != nil {