Do not reference original go-endpoints repo in the example.

Otherwise goop tool pulls original go-endpoints repo as a dependency.

TBR=dnj@chromium.org

samples/helloworld/helloworld.go

diff --git a/samples/helloworld/helloworld.go b/samples/helloworld/helloworld.go
index 7d31de6..db94506 100644
--- a/samples/helloworld/helloworld.go
+++ b/samples/helloworld/helloworld.go
@@ -18,7 +18,7 @@
 	"log"
 	"time"
 
-	"github.com/GoogleCloudPlatform/go-endpoints/endpoints"
+	"github.com/luci/go-endpoints/endpoints"
 	"golang.org/x/net/context"
 	"google.golang.org/appengine/datastore"
 )