tracing: update zipkin exporter import

tracing/tracing.go

@@ -13,7 +13,7 @@ import (
 
 	openzipkin "github.com/openzipkin/zipkin-go"
 	zipkinHTTP "github.com/openzipkin/zipkin-go/reporter/http"
-	"go.opencensus.io/exporter/zipkin"
+	"contrib.go.opencensus.io/exporter/zipkin"
 	"go.opencensus.io/plugin/ochttp"
 	"go.opencensus.io/trace"
 )